Fig. 23. Sections through the trunk of a scyllium embryo slightly younger than 28 F.
Figure A shews the separation of the body cavity from the pericardial cavity by a horizontal septum in which runs the ductus Cuvieri; on the left side is seen the narrow passage which remains connecting the two cavities. Fig. B through a posterior part of the trunk shews the origin of the segmental tubes and of the primitive ova.
sp.c. spinal canal; W. white matter of spinal cord; pr. commissure connecting the posterior nerve-roots; ch. notochord; x. subnotochordal rod; ao. aorta; sv. sinus venosus; cav. cardinal vein; ht. heart; pp. body cavity; pc. pericardial cavity; œs. solid œsophagus; l. liver; mp. muscle-plate; mp´. inner layer of muscle-plate; Vr. rudiment of vertebral body; st. segmental tube; sd. segmental duct; sp.v. spiral valve; v. subintestinal vein.
In the region of the head the mesoblast does not at first become divided into somites; but on the formation of the gill clefts a division takes place, which is apparently equivalent to the segmentation of the mesoblast in the trunk. This division causes the body cavity of the head to be divided up into a series of separate segments, one of which is shewn in [fig. 24], pp. The walls of the segments eventually give rise to the main muscles of the branchial clefts, and probably also to the muscles of the mandibular arch, of the eye, and of other parts. The cephalic sections of the body cavity will be spoken of as head cavities.
Fig. 24. Horizontal section through the last visceral arch but one of an embryo of Pristiurus.
ep. epiblast; vc. pouch of hypoblast which will form the walls of a visceral cleft; pp. segment of body-cavity in visceral arch; aa. aortic arch.
In addition to the parts already mentioned the mesoblast gives rise to the whole of the vascular system, and to the generative system. The heart is formed from part of the splanchnic mesoblast, and the generative system from a portion of the mesoblast of the dorsal part of the body cavity.
The hypoblast. Very shortly after the formation of the mesoblastic plates as lateral differentiations of the lower layer cells, an axial differentiation of the hypoblast appears, which gives rise to the notochord very much in the same way as in Amphioxus.
At first the hypoblast along the axial line forms a single layer in contact with the epiblast. Along this line a rod-like thickening of the hypoblast very soon appears ([fig. 25], B and C, Ch´) at the head end of the embryo, and gradually extends backwards. This is the rudiment of the notochord; it remains attached for some time to the hypoblast, and becomes separated from it first at the head end of the embryo ([fig. 25] A, ch): the separation is then carried backwards.
A series of sections taken through an embryo shortly after the first differentiation of the notochord presents the following characters.