On the hypothesis that the Sauropsidan embryos have come to occupy their central position, owing to an abbreviation of a process analogous to the linear closing of the blastopore behind the embryos of Elasmobranchii, all the appearances above described receive a satisfactory explanation. The passage at the front end of the primitive streak is the dorsal part of the blastopore, which in Elasmobranchii becomes converted into the neurenteric canal. The remainder of the primitive streak represents, in a rudimentary form, the linear streak in Elasmobranchii, formed by the coalesced edges of the blastoderm, which connects the hinder end of the embryo with the still open yolk blastopore. That it is in later stages not continued to the edge of the blastoderm, as in Elasmobranchii, is due to its being a rudimentary organ. The more or less complete fusion of the layers in the primitive streak is simply to be explained by this structure representing the coalesced edges of the blastopore; and the growth outwards from it of the mesoblast is probably a remnant of a primitive dorsal invagination of the mesoblast and hypoblast like that in the Frog. [FIG. 176. DIAGRAMMATIC LONGITUDINAL SECTION OF AN EMBRYO OF LACERTA. _pp._ body cavity; _am._ amnion; _ne._ neurenteric canal; _ch._ notochord; _hy._ hypoblast; _ep._ epiblast; _pr._ primitive streak. In the primitive streak all the layers are partially fused.] The final enclosure of the yolk in the Sauropsida takes place at the pole of the yolk-sack opposite the embryo, so that the blastopore is formed of three parts, (1) the neurenteric canal, (2) the primitive streak behind this, (3) the blastopore at the pole of the yolk-sack opposite the embryo. Mammalia. The features of the development of the placental Mammalia receive their most satisfactory explanation on the hypothesis that their ancestors were provided with a large-yolked ovum like that of the Sauropsida. The food-yolk must be supposed to have ceased to be developed on the establishment of a maternal nutrition through the uterus. On this hypothesis all the developmental phenomena subsequently to the formation of the blastodermic vesicle receive a satisfactory explanation. [FIG. 177. DIAGRAMMATIC LONGITUDINAL SECTION THROUGH THE POSTERIOR END OF AN EMBRYO BIRD AT THE TIME OF THE FORMATION OF THE ALLANTOIS. _ep._ epiblast; _Sp.c._ spinal canal; _ch._ notochord; _n.e._ neurenteric canal; _hy._ hypoblast; _p.a.g._ postanal gut; _pr._ remains of primitive streak folded in on the ventral side; _al._ allantois; _me._ mesoblast; _an._ point where anus will be formed; _p.c._ perivisceral cavity; _am._ amnion; _so._ somatopleure; _sp._ splanchnopleure.]