However, the early developmental sequence of these movements is not described in fine detail. In the present investigation, we show in detail the sequence of early differentiation of what will eventually become the individual brain parts from the precheliceral neuroectoderm of C.
This serves to complement and extend earlier studies of brain development in this species [ 27 ]. We show which areas of the precheliceral lobe the brain components are derived from, and clarify the separation of components from one another as development progresses.
In addition we describe three small 'pores' in the centre of the stomodeal anlage, similar to those observed in L. It is not currently understood what these pores are, but it has been proposed that their tri-radial construction may indicate a sister group relationship between pycnogonids sea spiders and chelicerates [ 40 ] and may therefore be of considerable phylogenetic value.
The tools available for the study of C. We also include more detailed information about the later developmental processes of the embryo. For example, in parallel with the early differentiation of the brain parts, the germ band elongates by addition of more segments, and the whole embryo goes through a process of complex tissue movements known as inversion [ 30 , 41 ]. This process results in the enclosure of the dorsal yolk mass into the body of the embryo, and is accomplished by the dorsal migration of the right and left halves of the germ band that eventually meet in the dorsal midline dorsal closure.
To facilitate the precise mapping and description of developmental events during inversion, we divide the process into four stages inversion I, II, III and dorsal closure.
Structures that undergo dramatic changes during inversion include the book lungs on the second opisthosomal segment and the tracheal system on the third opisthosomal segment, both of which are breathing organs. We observed fundamental differences in the sequence of appearance of the openings that give rise to the book lungs and the trachea, which we use to argue against the theory of serial homology of these organs [ 42 ]. The numbered stages into which we divide development in C.
Besides a number, each stage is also given a colloquial name for practical use. The number of stages allocated to early development up to stage 6 is higher in the Seitz system, where the early stages are separated into numerous time hour intervals after egg laying hAEL.
The resolution we chose for the early stages reflects the degree of detail we were able to observe using our methods, and follows the staging nomenclature of A. From stage 6 to 10, the staging resolution of the two systems is similar, and after stage 10 our new system is more detailed additional file 1.
Overview of development of the embryo, postembryo and first and second instar of C. Nuclear stained embryos illustrate the 21 embryonic stages. The postembryo and the first and second instar are illustrated by images of live animals. Additional file 1 provides a comparison between this revised staging system and the system used in earlier publications. During the process of egg-laying, the female spider produces a liquid secretion that guides the soft and ovoid-shaped eggs from the genital opening into a silk pouch that is then formed by the female into a round cocoon [ 7 ].
The liquid secretion is absorbed by the eggs, which as a result increase in size, become more solid and take on a spherical shape of roughly 1. During these first hours of development the eggs are sticky and fragile. Because of these conditions, we waited 12 to 24 hours before it was possible to remove intact single eggs for further investigation.
Eggs at stage 1 largely consist of yolk, which is distributed as fine homogeneous granules. In these early stages, the nuclei are surrounded by a mass of yolk and not yet enclosed within cell membranes. The first cycles of nuclear division are superficial nuclear mitosis without cytokinesis which results in a polynuclear cell.
The divisions take place intralecithally in the centre of the egg Figures 2a, b. Based upon earlier observations [ 26 ] we assume that the first cleavage cycles are synchronous. During these cleavages the nuclei start to migrate towards the egg surface. Stages of C. Sytox staining, a-c , e , h-j ; light micrograph, d ; SEMs, f , g.
Due to the large amount of yolk, the nuclei in the egg centre are not visible. Nuclei white arrows are visible within the broken yolky mass Y.
The periplasm is the primordial cytoplasm surrounding the nuclei and is lobate in shape white lines at the surface of the egg. The yolky mass is now organized into large pyramid-shaped compartments white dotted line. After a few division cycles, their cytoplasm attains a more rounded shape at movie frame of Additional file 2.
This is when the cleavage type changes from superficial to holoblastic, and each nucleus on the egg surface is surrounded by its cell membrane Figures 2e, f.
A layer of early blastodermic cells is now evenly distributed over the egg surface Figures 2e, f. These nuclei probably belong to vitellophages; multi-nucleic cells which phagocytize the intracellular yolk. It is currently unclear whether all of these vitellophages have their origin in the early blastoderm secondary immigration or whether some of them derive from inside the egg [ 30 , 43 ].
However, it is likely that during cell migration from the egg centre to the surface, some cells remain in the yolk mass and do not reach the surface.
The following cell divisions are asynchronous. Because of the frequency of cell division cycles, the egg appears to contract e. After roughly three cell division cycles, the contractions subside, leaving the blastodermic cells still more or less evenly distributed over the egg surface.
There is no formation of a germ disc a dense aggregation of cells that provides the primordial tissue for the embryo body and is commonly present in arthropod embryos [ 30 ]. Cells appear to migrate inwards at the blastopore Figures 2h, i ; at movie frame of Additional file 3 and initiate gastrulation- the developmental process that results in a layer of mesendoderm beneath the surface layer of blastoderm now ectoderm.
In the egg hemisphere that contains the advanced blastopore, scattered divisions result in an uneven distribution of the blastodermic cells. The blastopore region displays high nuclear density with the nuclei arranged in several layers. This indicates the end of gastrulation, i. As development proceeds, the cellular tissue of the region where the blastopore formed becomes thicker and appears to bulge outwards visible around movie frame of Additional file 2.
This conspicuous structure is known as the primary thickening, or cumulus anterior [ 25 , 31 , 32 ]. Sytox staining, a-d , f-g ; light micrograph, e. It bulges slightly outwards, a phenomenon which in this and subsequent figures is called primary thickening PT. The surrounding nuclei are arranged in irregular patches arrows. At this pole of the egg, the nuclei are more evenly distributed. The cumulus Cu has reached its final position after separation from the primary thickening PT.
The migration of cells from the primary thickening beneath the surface layer has resulted in one hemisphere with greater cell density top of photo e compared with the other hemisphere bottom of photo e. The line of demarcation between the dense hemisphere and the less dense one is the 'equator'. This marked difference in cell density is evident between the two arrowheads in e. The tissue between the cumulus and the primary thickening spreads laterally, forming a region of low cell density called the dorsal field DF.
This region will continue to have much yolk in further stages of development. The primary thickening PT is at the posterior edge of the developing embryo body and located in the hemisphere with greater cell density.
The hemisphere with lower cell density is extra-embryonic Ee since it continues as a yolk-filled region. The dorsal field DF expands to about degrees in width, and the cumulus has disappeared. From the primary thickening formerly the blastopore clusters of internalised cells migrate in radial directions.
Two different types of cellular movement are observed: The most prominent movement follows the asymmetrical fission of the primary thickening into two cell groups visible in Additional file 2 at about movie frame and in Additional file 3 at movie frame A smaller group remains at the centre of the embryonic portion of the egg and continues to be identified as primary thickening.
The larger cell group, the cumulus or cumulus posterior, is visible as a little bulge on the egg surface Figures 3c-e ; Additional files 2 and 3. The cumulus migrates about 90 degrees underneath the egg surface. The second type of cellular movement is a radial migration of single cells or groups of a few cells starting from the primary thickening. These cells primordial mesendoderm migrate directly underneath the ectodermal layer up to 90 degrees along the outer egg curvature Additional files 2 and 3.
The embryo now has two dissimilar egg hemispheres. The embryonic hemisphere appears to be more opaque because of the new layer of cells primordial mesendoderm underneath the ectoderm. The more translucent hemisphere is made up of extra-embryonic tissue, mainly filled with yolk in this and subsequent figures e. Stages 7 and 8 of C. Sytox staining, a-g ; SEM, h.
The dorsal field DF is extended to its maximum. The embryonic tissue early germ band extends along the ventral curvature between the two arrows. Between the two white arrow heads is the equator, which is the sharp change in cell density that marks the border of migration of mesendodermal cells from the primary thickening.
The embryonic tissue early germ band extends along the ventral curvature between the two white arrows. This region has more cells and a much higher density than the dorsal field DF. The equator is no longer visible. Evident are all future prosomal segments: cheliceres Ch , pedipalps P and four walking legs white arrow heads. At the posterior end, the growth zone GZ exhibits a higher density of cells.
Anterior to the cheliceral segment Ch the precheliceral region Pc is separated by a clear margin from the surrounding extra-embryonic mainly yolk tissue. All prosomal segments cheliceres, Ch; pedipalps, P; four walking legs, L1-L4 are distinct. An embryo comparable to e. Evident are the germ band GB and yolk Y. The yolk is located in the regions labelled earlier as the dorsal field DF and the extra-embryonic Ee region. Stage 9, Prosomal limb buds. Sytox staining, a-a'' ; SEMs, b, c.
The dorsal field DF in earlier figures is probably now a mass of extra- and intra-cellular yolk Y. Between the developing appendages the ventral sulcus VS is visible as a narrow length of midline tissue with a low cell density compared with the bilateral appendage regions. The white arrows show that the precheliceral region Pc extends anteriorly from the anterior base of the cheliceres. The dotted lines indicate the progress of opisthosomal segment formation anterior to the growth zone GZ.
The white dotted lines indicate the lateral margins of the precheliceral region Pc. Limb buds are barely evident at this stage on opisthosomal segments O1-O3. The white dotted lines indicate the progress of the formation of additional opisthosomal segments.
Stage 10, Prosomal limb bud elongation. Sytox staining, a-a'' ; SEMs, b-d. Limb buds of pedipalps P and walking legs L1-L4 are prominent in the prosoma, and segments are clearly distinguishable in the opisthosoma e. The white arrows show that the precheliceral region Pc extends to the posterior base of the cheliceres Ch. The ventral sulcus VS is a thin length of tissue between the germ band halves. Five segments O1-O5 with paired bulges are visible in the opisthosoma.
A developing sixth opisthosomal segment white arrowhead is evident anterior to the growth zone GZ. The white dotted line indicates the posterior border of the precheliceral region Pc. The opisthosomal segment two O2 has a clear limb bud while opisthosomal segments three and four O3, O4 only show the beginning formation of the limb bud white arrowheads. This stage is characterised by a major rearrangement of embryonic tissue.
This region is relatively translucent and is made up of far fewer cells than the ventral area where the embryo body will differentiate. Our data does not show whether the dorsal field is formed exclusively by cell migration or whether cell death is also involved. The primary thickening former blastopore region will become the posterior end of the embryo body.
At this stage, it is not possible to determine the axis orientation of unstained eggs because of a lack of visible landmarks. When nuclear staining is applied, however, a germ band becomes evident Figures 4a-d. This latter region is now called the growth zone GZ as in Figures 4a-d. As the germ band becomes visible and gradually lengthens along the ventral curvature, the equator disappears and the dorsal field expands laterally.
The widening of the dorsal field is evident white dotted lines in Figures 4b-d. At the end of this 'Germ band' stage, the embryonic tissue has lengthened anteriorly and covers the entire ventral surface of the embryonic region. In this ventral region of the embryo, the cell density is much greater than in the dorsal field. Within the spherical eggshell, the embryo now has the shape of a flattened ovoid. The C-shaped germ band invariably lies along the longitudinal egg axis Figures 4e-h.
The developing embryo body embryo proper of primordial segments and appendages is very dense, comprising many more cells than the dorsal extra-embryonic tissue Figures 4e-g.
The dense region of cells just anterior to the cheliceres initially has no organized structures Figures 4e, f and the border between this region and the extra-embryonic region is less defined than the perimeter of the rest of the embryo body. The cheliceral segment is slightly smaller than the posterior segments Figures 4e-g. At its posterior end, the embryo proper has a growth zone GZ; Figures 4e, g, h which appears brighter in nuclear staining. The growth zone is rounded posteriorly and has a higher cell density than the remaining embryo proper.
In the precheliceral region there is often a slight difference in the progression of development of the right and left halves e. Figure 5a'. The posterior margins of the precheliceral lobes appear anterior to the cheliceral segment but will gradually extend posteriorly white arrows; Figures 5a', b.
All the prosomal segments are more prominent than in the previous stage, and the prosomal limb buds cheliceres, pedipalps and four walking legs bulge outward. The buds are broad and flat and point in a postero-ventral direction Figures 5a, a', b. The cheliceral buds are smaller and slightly more medial than the buds of the pedipalps and walking legs Figures 5a, a', b. The opisthosoma has about four visible segment anlagen Figures 5a, a'', c. The growth zone has a posterior curvature and is less broad than the more anterior segments Figures 5a'', c.
For stages we adhere to the staging system defined for A. However, we deviate from this system at stage 10 as this stage is not well described for A. Furthermore, stage 10 for A. We define a new stage 10 and new subsequent stages for C. The precheliceral region is broader than the remaining parts of the germ band Figures 6a', b. The posterior margins of the precheliceral region have moved anterior to the pedipalpal segment and now enclose the cheliceral segment Figures 6a', b.
None of the prosomal limb buds are segmented yet, and they vary in their width-to-length ratio Figures 6a-c. The slightly depressed ventral sulcus VS extends from the centre of the precheliceral region to opisthosomal segment three Figures 6a', c. It is at this point considerably smaller than the subsequent opisthosomal segments. In Figure 6a'' a developing sixth opisthosomal segment white arrow head is evident anterior to the growth zone.
At this stage, the precheliceral region is partitioned medially into bilateral precheliceral lobes Figures 7a', b. The anlage of the stomodeum becomes visible. Stage 11, Opisthosomal limb buds. Sytox staining, a-a'' ; SEMs, b-e. The single precheliceral area Pc, Figure 6 is now divided into bilateral precheliceral lobes PcL.
Laterally adjacent to the ventral sulcus, segmentally iterated point-like depressions are evident black arrow heads. This is presumably neural precursor tissue. The ventral sulcus VS is prominent between the developing limb buds in the prosoma and opisthosoma. Each of these lobes has evenly distributed point-like depressions some indicated by black arrow heads.
The white arrow shows the postero-medial furrow of the forming stomodeum. Laterally adjacent to it are two conspicuous point-like neural depressions white arrow heads. At the pedipalps P , the anlage of an endite en is evident as a proximo-medial swelling. Prosomal limb buds L appear three-segmented, and in between the limb buds is a larger invaginating region black arrows of presumptive neural precursor cells. Barely visible are the remnants of limb buds white arrow head on opisthosomal segment one O1.
Seven clearly separated opisthosomal segments O1-O7 are visible while an additional segment vertical dotted line is still connected with the growth zone GZ. The ventral sulcus VS, horizontal white dotted lines extends posteriorly black arrow as the limb buds become differentiated e : At the medio-posterior base of the limb buds of opisthosomal segments two and three O2, O3 are conspicuous depressions white arrows made up of primordial tissue for the respiratory system.
The white dotted line indicates the right boundary of the ventral sulcus VS. Ch: chelicere, P: pedipalp. The cheliceral limb buds have become more flattened and are approximately twice as long as they are wide. They have twisted slightly ventro-posteriorly, and their distal parts are cone-like Figures 7a', b. The ventral sulcus has extended posteriorly, reaching the sixth opisthosomal segment in older embryos of this stage Figures 7a'', d.
Small spots black arrow heads; Figures 7a', a'' in a segmentally iterated pattern are barely visible laterally adjacent to the ventral sulcus. This area is the ventral neuroectoderm and the point-like depressions correspond to neural precursor tissue [ 27 ]. The pedipalps and walking legs continue elongation and start to bend ventrally. For a short time, a small structure is visible on the first opisthosomal segment.
With a developmental gradient from anterior more developed to posterior less developed , primordial limb buds have appeared as small bulges on opisthosomal segments two to five Figures 7a, a'', d. The initial shape of these buds is not as broad as the prosomal limb buds when they first appeared compare with stage 9; Figures 5a', b. These invaginations are precursor tissue for the book lung system.
At least seven separated segments are visible anterior to the growth zone Figures 7a'', d. The outer edges of the precheliceral lobes have become very distinct, and the lobes stand out clearly from the surrounding tissue Figures 8a', b. Alongside the point-like depressions, a slight relief begins to form on the precheliceral lobes.
Lateral to the stomodeum, invaginating kidney shaped folds of neural tissue are visible lateral furrow LF; Figures 8a', b. Medially adjacent to these folds a minimal elevation can be seen in some specimens.
Stage 12, Lateral furrow. Sytox staining, a-a''; SEMs, b , c. Lateral to the stomodeum Sto are invaginating folds of precursor neural tissue LF, lateral furrow. The pedipalps P , cheliceres Ch , and stomodeum are more pronounced. The walking legs visible L4 are more elongated and their tips touch each other.
The ventral sulcus VS, white dotted line extends posteriorly almost the eighth opisthosomal segment O8. Anterior to the stomodeum Sto , the small bi-lobed anlage of the labrum Lb is visible. Postero-laterally on each precheliceral lobe PcL , a kidney shaped lateral furrow LF is evident. Eight separate opisthosomal segments O1-O8 are visible whilst an additional segment indicated by a white dotted line is still connected to the growth zone GZ.
Small depressions black arrows of the primordial respiratory system are evident at the posterior insertion of the limb bud at opisthosomal segment two O2. The ventral sulcus VS, black dotted line extends posteriorly to the seventh opisthosomal segment O7.
The prosomal limbs pedipalp and walking legs have elongated, and the tips of the walking legs from each body halve approach each other. The pedipalps and all the walking limbs display signs of annulations. It is not clear how these annulations relate to later leg segments.
The ventral sulcus extends posteriorly to the seventh opisthosomal segment, and has slightly widened Figures 8a'', c. All opisthosomal limb buds have a globular shape Figures 8a'', c. Medially and between the opisthosomal limb buds, large point-like depressions of neural precursor tissue can be seen. Up to eight separate opisthosomal segments are visible anterior to the growth zone Figures 8a'', c. Two distinct fields of neural precursor tissue are evident within the crescent-shaped precheliceral lobes: the medial subdivision and the lateral subdivision ms and ls, sensu [ 37 ].
Figure 12 b. Stage 13, Labrum. Sytox staining, a-a'' ; SEMs; b-d. Cheliceres Ch , pedipalps P and the four walking limbs L1-L4 are more prominent in the prosoma. Because of the growth that has taken place along the ventral curvature, the posterior end of the opisthosoma approaches white line the anterior border of the precheliceral lobes PcL. Anterior to the stomodeum Sto the prominent labrum Lb is evident.
Cheliceres Ch and pedipalps P are more pronounced. The ventral sulcus VS, white dotted line extends posteriorly to the eighth opisthosomal segment O8. Two distinct fields of precursor neural tissue are evident within the crescent-shaped precheliceral lobes: the medial ms and lateral ls subdivisions.
The cheliceres Ch have a proximal base bs and a distal fang f. The pedipalp P is four-segmented and bears an endite en on its first segment. The white arrowheads show that each walking leg L1-L4 is subdivided into five podomeres There is a point-like depression black arrows of what is presumed to be neural precursor tissue at the distal tip of each leg.
Nine opisthosomal segments O1-O9 are visible while an additional segment is still connected to the growth zone GZ. Black arrowheads indicate segmental furrows that mark the boundary between opisthosomal segments. A slit-like invagination black arrow, primordial respiratory tissue is evident at the posterior base of the limb bud at opisthosomal segment two O2.
The ventral sulcus black dotted line extends posteriorly to the eighth opisthosomal segment O8. Stage 16, Inversion III. Sytox staining, a-a'' , c ; SEMs, b , d, e. The white line indicates the increased distance from the precheliceral lobes PcL to the opisthosomal tail compared to previous stages compare with Figure 10 a and 11 a. The prosomal tergites start to extend dorsally white arrows. The white dotted line indicates the more posterior position of the mouth opening in relation to the lateral subdivision of the brain compare with Figures 10 a' and 11 a'.
The white dotted line shows the progress of inversion see the lower diagram in Figure 11 d which schematically illustrates inversion. The medial subdivision ms is growing anteriorly black arrows , partially covering the anterior furrow AF. The lateral furrow LF is totally covered by tissue from the lateral subdivision ls. The mouth opening is covered by the medially enlarged tip of the labrum Lb.
Anlagen of the segmental sternites Ste, white dotted line are evident medial to the pedipalps P and walking legs L1.
At the posterior base of the limb bud on opisthosomal segment two O2 , three pulmonary furrows black arrows and a lateral opening of the pulmonary sac PuS are evident. At the latero-posterior insertion of the limb bud on opisthosomal segment three O3 , the opening of the tubular trachea TrO is visible. The globular limb bud on opisthosomal segment four O4 will differentiate into the anterior spinneret ASp , whereas the dorso-ventrally elongated limb bud on opisthosomal segment five O5 will differentiate into the posterior PSp and medial MSp spinnerets.
Eight opisthosomal segments O4-O11 are clearly evident here. On the dorsal surface, the primordial tergite plates Ter have further expanded compare with the later stage in Figure 14 c. Between the eleventh opisthosomal segment O11 and the growth zone GZ , small bilateral lobes probably represent the twelfth opisthosomal segment O12?
Ch, chelicere; Lab, labium; Te, telson. The pedipalps and walking legs show clearer annulations and a subdivision into podomeres is evident. The pedipalps are divided into four segments, and the walking legs have five segments Arabic numbers in Figures 9b, c. All opisthosomal limb buds retain their globular shape. The fifth opisthosomal limb buds are still smaller than the more anterior buds.
The opisthosoma has up to nine separated segments and the ventral sulcus, which has again slightly widened, extends posteriorly to the eighth opisthosomal segment Figures 9a, d. The gradual widening of the ventral sulcus, which from stage 11 to 13 is a relatively slow process, significantly accelerates during stage 14 Figures 10a'', e.
This marks the start of inversion, a complex sequence of tissue movement and growth that results in a rearrangement of the body and incorporation of the yolk mass into the embryo. Apart from the precheliceral region and the posterior-most opisthosomal segments, the two halves of the germ band move separately over the yolk mass until they connect again on the dorsal side Figure 11d gives a schematic overview. As a result of this movement, the distance between the precheliceral region and the posterior opisthosomal region increases.
Simultaneously, the germ band continues to extend with the addition of the final opisthosomal segments. The precheliceral region, which until inversion was an extension of the rest of the germ band, gradually folds posteriorly. In order to precisely map the various developmental events that occur during inversion, we distinguish four separate stages.
Stage 14, Inversion I. Sytox staining, a-a'' , d ; SEMs, b , c , e , f. The distance between the posterior opisthosoma and the anterior border of the precheliceral lobes PcL has increased indicated by white line, compare with Figure 9 a. Between the precheliceral lobes PcL the stomodeum Sto has moved posteriorly. The white dotted line shows the more anterior position of the mouth opening in relation to the lateral subdivision indicated by white arrows of the brain.
The white dotted line shows the progress of inversion see the upper diagram in Figure 11 d which schematically illustrates inversion. Anterior to the medial subdivision ms , the anterior furrow AF has formed. The anterior furrow has also been termed semi-lunar or cerebral groove in other arachnids [e. The lateral subdivision ls migrates black arrows in the direction of the lateral furrow LF , partly covering it. The mouth opening is surrounded anteriorly by the labrum Lb and posteriorly by the labium Lab.
The ectodermal tissue medial to the prosomal limbs shows a grid-like formation of black spots white arrows , presumably primordial neural tissue. The black line indicates the relative progress of the ventral sulcus VS. Nine separate opisthosomal segments are present. The black dotted line indicates an additional segment anterior to the growth zone GZ.
The black asterisks designate lobes of anlagen that will eventually become tergites on the dorsal surface of the body. At the lateral base of the limb buds of opisthosomal segment two O2 , the opening of the pulmonary sac white arrow can be seen. Medially adjacent to it are two slit-like openings black arrows to the developing book lungs.
Ch, chelicere; L1-L4, walking legs one to four; P, pedipalp. Stage 15, Inversion II. Sytox staining, a-a'' ; SEMs, b , c. The white line indicates the increased distance from the precheliceral lobes PcL to the opisthosomal tail compared with previous stages see Figure 10 a. The white dotted line indicates the mouth opening between the two lateral subdivisions of the developing brain compare with Figure 10 a'.
The two labral lobes have completely fused and the labrum Lb is now an unpaired structure. The white dotted line shows the progress of inversion middle diagram in d. Separated opisthosomal segments four to nine O are visible. The tenth O10 and the future eleventh segments black dotted line are located together with the growth zone GZ in a tail-like portion of the germ band that protrudes from the mass of yolk. Small bulges of tergite anlagen Ter are evident on the dorsal surface compare with the more differentiated tergite anlagen in Figure 14 c.
At the posterior base of the limb bud of O2 the opening of the pulmonary sac white arrow is seen, and adjacent to it medially are two slit-like openings black arrows to the book lungs. The podomeres of the fourth walking leg L4 are numbered from base to tip. Posterior view, dorsal is at the top of the diagrams. The germ band brown areas has divided, and the ventral sulcus VS is increasing in width. The bilateral regions of the germ band are migrating dorsally black arrows , enclosing the yolk area Y and eventually meeting in the dorsal midline stage 17, dorsal closure.
By stage 15, the dorsal edges of both halves of the germ band lie in a line when viewed from posterior. AF, anterior furrow; Ch, chelicere; P, pedipalp; X, damaged area, cuticle torn. At Inversion I, the dorsal edges of the body halves have not yet reached the upper hemisphere of the egg. The precheliceral lobes are characterized by a high density of point-like depressions and even more pronounced anterior rims Figures 10a', b.
In addition, the medial and lateral subdivisions are more evident. Anterior to the medial subdivision , a crescent shaped anterior furrow has formed AF, Figure 10b. The anterior furrow has also been termed the semi-lunar or cerebral groove in other arachnids [e. The lateral subdivision migrates in the direction of the lateral furrow , partly covering it black arrows; Figure 10b. The cheliceres are now two-segmented. The proximal segment basal segment widens distally and the tapering distal segment fang sits slightly off-centre on the basal segment Figure 10c.
This widening is probably related to anterior and posterior invagination sites on each of these leg segments. The neuroectoderm medial to the prosomal limbs displays a grid-like formation of point-like depressions white arrows; Figure 10d. The buds on opisthosomal segment two have become dorso-ventrally elongated.
On the posterior ends of these buds, the opening of the pulmonary sac and one or two pulmonary furrows are evident Figures 10e, f. The buds on opisthosomal segments three to five are undifferentiated and still more or less globular in shape.
Dorsal to the opisthosomal limb buds, the anlagen of the tergite plates are evident black stars; Figure 10e.
At the posterior end of the embryo, nine opisthosomal segments have separated from the growth zone Figures 10a, e. The growth zone now protrudes slightly from the yolk, marking the start of the tail-like formation of the 'post-opisthosoma' name derived from ' Postabdomen' [ 46 ]. The opisthosomal body halves have reached the dorsal hemisphere of the egg and form a line when viewed from a caudal perspective white dotted line; Figure 11a''.
The two labral lobes have completely fused and the labrum is now an unpaired structure Figure 11a'. The labrum and stomodeum have jointly started the posterior migration that will be continued in subsequent stages.
By stage 15, the cleft between the two precheliceral lobes has become deeper, and the mouth opening lies between the lateral subdivisions on both head lobes white dotted line; Figure 11a'. The labrum now partially covers the stomodeum Figure 11b. Ten separate opisthosomal segments are evident anterior to the growth zone Figures 11a'', b.
The posterior base of the limb bud on opisthosomal segment two bears two pulmonary furrows black arrows and the lateral opening of the pulmonary sac white arrow; Figure 11c. The tenth and the future eleventh opisthosomal segments are now forming, together with the growth zone of the tail-like post-opisthosoma Figure 11b. Small bulges of tergite anlagen are evident on the dorsal surface of the opisthosomal segments Figure 11b.
By the third stage of inversion, the tergite plates of the opisthosoma are completely enclosed within the dorsal hemisphere of the egg: from a caudal perspective the opisthosomal limb buds of both halves lie more or less in one line Figures 12a'' , 11d. The distance between the precheliceral lobes and post-opisthosoma has increased and is about a quarter of the total circumference of the embryo Figure 12a. The lateral furrows are completely covered by tissue from the kidney-shaped lateral subdivisions Figure 12b.
The medial subdivisions are growing anteriorly, partially covering the anterior furrows black arrows; Figure 12b. The anterior furrows are partially closed by anterior expansions of medial subdivisions Figure 12b.
The tip of the labrum is stretched medially and points in a ventral direction Figures 12a', b. Posterior to the stomodeum, the unpaired anlage of the labium is formed Figure 12b. The bases of the cheliceres have further widened, and at the ventral base of the pedipalp a prominent endite is evident Figure 12b. Triploblasty can be seen in multicellular animals, particularly, flatworms Phylum Platyhelminthes , mollusks Phylum Mollusca , arthropods Phylum Arthropoda , and chordates Phylum Chordata.
Animals that are not triploblastic are some invertebrates like sponges Phylum Porifera and cnidarians Phylum Cnidaria. Triploblasts are organisms whose body is derived from all three germ layers, which are the endoderm, mesoderm, and ectoderm.
Due to the interface of ectoderm and endoderm, differentiation of mesodermal cells takes place. The mesoderm forms the coelom, which is the body cavity. These organisms are complex due to the presence of a body cavity. Different organs are present in the body cavity. These organs are protected by a fluid that is present in the surrounding. In the coelom, fluid cushions protect the organs against any shock. Sabhadiya, The organs of the body are protected against dehydration and shock due to this fluid.
In higher animals, the mesoderm is a distinguishing feature as it forms lungs, liver, stomach, colon, urinary bladder, and other body organs. From flatworms to humans, all animals are triploblastic.
Humans are the supreme example of triploblastic animals. Triploblastic evolutionary development: Scientists thought that about years ago, triploblastic organisms triploblastic were developed from diploblastic organisms. The difference between the diploblastic and triploblastic organisms is discussed here further below. Genes are the blueprint of our bodies, a blueprint that creates a variety of proteins essential to any organism's surviv..
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There are more species of insects than any other species combined. These animals are called diploblasts , and have a nonliving middle layer between the endoderm and ectoderm although individual cells may be distributed through this middle layer, there is no coherent third layer of tissue. The four clades considered to be diploblastic have different levels of complexity and different developmental pathways, although there is little information about development in Placozoa.
More complex animals usually those with bilateral symmetry develop three tissue layers: an inner layer endoderm , an outer layer ectoderm , and a middle layer mesoderm.
Animals with three tissue layers are called triploblasts. Figure 1. Diploblastic and triploblastic embryos. During embryogenesis, diploblasts develop two embryonic germ layers: an ectoderm and an endoderm or mesendoderm.
Triploblasts develop a third layer—the mesoderm—which arises from mesendoderm and resides between the endoderm and ectoderm. Each of the three germ layers is programmed to give rise to specific body tissues and organs, although there are variations on these themes. Generally speaking, the endoderm gives rise to the lining of the digestive tract including the stomach, intestines, liver, and pancreas , as well as to the lining of the trachea, bronchi, and lungs of the respiratory tract, along with a few other structures.
The ectoderm develops into the outer epithelial covering of the body surface, the central nervous system, and a few other structures. The mesoderm is the third germ layer; it forms between the endoderm and ectoderm in triploblasts. This germ layer gives rise to all specialized muscle tissues including the cardiac tissues and muscles of the intestines , connective tissues such as the skeleton and blood cells, and most other visceral organs such as the kidneys and the spleen.
Diploblastic animals may have cell types that serve multiple functions, such as epitheliomuscular cells, which serve as a covering as well as contractile cells. Further subdivision of animals with three germ layers triploblasts results in the separation of animals that may develop an internal body cavity derived from mesoderm, called a coelom , and those that do not.
This epithelial cell-lined coelomic cavity , usually filled with fluid, lies between the visceral organs and the body wall. It houses many organs such as the digestive, urinary, and reproductive systems, the heart and lungs, and also contains the major arteries and veins of the circulatory system.
In mammals, the body cavity is divided into the thoracic cavity, which houses the heart and lungs, and the abdominal cavity, which houses the digestive organs.
In the thoracic cavity further subdivision produces the pleural cavity, which provides space for the lungs to expand during breathing, and the pericardial cavity, which provides room for movements of the heart. The evolution of the coelom is associated with many functional advantages. For example, the coelom provides cushioning and shock absorption for the major organ systems that it encloses.
In addition, organs housed within the coelom can grow and move freely, which promotes optimal organ development and placement. The coelom also provides space for the diffusion of gases and nutrients, as well as body flexibility, promoting improved animal motility.
Triploblasts that do not develop a coelom are called acoelomates , and their mesoderm region is completely filled with tissue, although they do still have a gut cavity. Examples of acoelomates include animals in the phylum Platyhelminthes, also known as flatworms. Animals with a true coelom are called eucoelomates or coelomates Figure 2. In such cases, a true coelom arises entirely within the mesoderm germ layer and is lined by an epithelial membrane.
This membrane also lines the organs within the coelom, connecting and holding them in position while allowing them some freedom of movement. Annelids, mollusks, arthropods, echinoderms, and chordates are all eucoelomates. A third group of triploblasts has a slightly different coelom lined partly by mesoderm and partly by endoderm.
The phylum Nematoda roundworms is an example of a pseudocoelomate. True coelomates can be further characterized based on other features of their early embryological development. Figure 2. Body cavities. Triploblasts may be a acoelomates, b eucoelomates, or c pseudocoelomates. Acoelomates have no body cavity. Eucoelomates have a body cavity within the mesoderm, called a coelom, in which both the gut and the body wall are lined with mesoderm.
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