The parts derived from the two layers of the mesoblast (not including special organs or the vascular system) are as follows:-- From the somatic layer are formed (1) A considerable part of the voluntary muscular system of the body. (2) The dermis. (3) A large part of the intermuscular connective tissue. (4) Part of the peritoneal epithelium. From the splanchnic layer are formed (1) A great part of the voluntary muscular system. (2) Part of the intermuscular connective tissue. (3) The axial skeleton and surrounding connective tissue. (4) The muscular and connective-tissue wall of the alimentary tract. (5) Part of the peritoneal epithelium. [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; _oes._ solid oesophagus; _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.