_df._ dorsal fin; _sp.c._ spinal cord; _pp._ body cavity; _sp._ splanchnic layer of mesoblast; _so._ somatic layer of mesoblast; _mp´._ commencing differentiation of muscles; _ch._ notochord; _x._ subnotochordal rod arising as an outgrowth of the dorsal wall of the alimentary tract; _al._ alimentary tract.] The undivided ventral portion gives rise to the general somatic and splanchnic mesoblast (fig. 185), and the cavity between its two layers constitutes the body cavity. The originally separate halves of the body cavity eventually meet and unite in the ventral median line throughout the greater part of the body, though in the tail they remain distinct and are finally obliterated. Dorsally they are separated by the mesentery. From the mesoblast at the junction of the dorsal and ventral parts of the primitive plates is formed the urinogenital system. That the above mode of origin of the mesoblast and notochord is to be regarded as a modification of that observable in Amphioxus seems probable from the following considerations:-- In the first place, the mesoblast is split off from the hypoblast not as a single mass but as a pair of distinct masses, comparable with the paired diverticula in Amphioxus. Secondly, the body cavity, when it appears in the mesoblast plates, _does not arise as a single cavity, but as a pair of cavities, one for each plate of mesoblast_; and these cavities remain permanently distinct in some parts of the body, and nowhere unite till a comparatively late period. Thirdly, the primitive body cavity of the embryo is not confined to the region in which a body cavity exists in the adult, _but extends to the summit of the muscle-plates_, at first separating parts which become completely fused in the adult to form the great lateral muscles of the body. [FIG. 185. SECTION THROUGH THE TRUNK OF A SCYLLIUM EMBRYO SLIGHTLY YOUNGER THAN 28 F. _sp.c._ spinal canal; _W._ white matter of spinal cord; _pr._ posterior nerve-roots; _ch._ notochord; _x._ subnotochordal rod; _ao._ aorta; _mp._ muscle-plate; _mp´._ inner layer of muscle-plate already converted into muscles; _Vr._ rudiment of vertebral body; _st._ segmental tube; _sd._ segmental duct; _sp.v._ spiral valve; _v._ subintestinal vein; _p.o._ primitive generative cells.] It is difficult to understand how the body cavity could thus extend into the muscle-plates on the supposition that it represents a primitive split in the mesoblast between the wall of the gut and the body-wall; but its extension to this part is quite intelligible, on the hypothesis that it represents the cavities of two diverticula of the alimentary tract, from the muscular walls of which the voluntary muscular system has been derived; and it may be pointed out that the derivation of part of the muscular system from what is apparently splanchnic mesoblast is easily explained on the above hypothesis, but not, so far as I see, on any other.