The formation of the blastodermic vesicle may perhaps be explained on the view that in the Proto-mammalia the yolk-sack was large, and that its blood-vessels took the place of the placenta of higher forms. On this view a reduction in the bulk of the _ovarian ovum_ might easily have taken place at the same time that the presence of _a large yolk-sack_ was still necessary for the purpose of affording surface of contact with the uterus. _The formation of the Mesoblast and of the Notochord._ [FIG. 180. SECTIONS OF AN AMPHIOXUS EMBRYO AT THREE STAGES. (After Kowalevsky.) A. Section at gastrula stage. B. Section of an embryo slightly younger than that represented in fig. 169 D. C. Section through the anterior part of an embryo at the stage represented in fig. 169 E. _np._ neural plate; _nc._ neural canal; _mes._ archenteron in A and B, and mesenteron in C; _ch._ notochord; _so._ mesoblastic somite.] Amphioxus. The mesoblast originates in Amphioxus, as in several primitive invertebrate types, from a pair of lateral diverticula, constricted off from the archenteron (fig. 180). Their formation commences at the front end of the body and is thence carried backwards, and each diverticulum contains a prolongation of the cavity of the archenteron. After their separation from the archenteron the dorsal parts of these diverticula become divided by transverse septa into successive somites, the cavities of which eventually disappear; while the walls become mainly converted into the muscle-plates, but also into the tissue around the notochord which corresponds with the vertebral tissue of the higher Chordata. The ventral part of each diverticulum, which is prolonged so as to meet its fellow in the middle ventral line, does not become divided into somites, but contains a continuous cavity, which becomes the body cavity of the adult. The inner layer of this part forms the splanchnic mesoblast, and the outer layer the somatic mesoblast. The notochord would almost appear to arise as a third median and dorsal diverticulum of the archenteron (fig. 180 _ch_). At any rate it arises as a central fold of the wall of this cavity, which is gradually constricted off from before backwards. Urochorda. In simple Ascidians the above processes undergo a slight modification, which is mainly due (1) to a general simplification of the organization, and (2) to the non-continuation of the notochord into the trunk. [FIG. 181. TRANSVERSE OPTICAL SECTION OF THE TAIL OF AN EMBRYO OF PHALLUSIA MAMMILLATA. (After Kowalevsky.) The section is from an embryo of the same age as fig. 8 IV. _ch._ notochord; _n.c._ neural canal; _me._ mesoblast; _al´._ hypoblast of tail.]