The whole of the blastodermic vesicle, except the embryonic area, represents the yolk-sack, and the growth of the hypoblast and then of the mesoblast round its inner wall represents the corresponding growths in the Sauropsida. As in the Sauropsida it becomes constricted off from the embryo, and the splanchnopleuric stalk of the sack opens into the ileum in the usual way. [FIG. 178. OPTICAL SECTIONS OF A RABBIT'S OVUM AT TWO STAGES CLOSELY FOLLOWING UPON THE SEGMENTATION. (After E. van Beneden.) _ep._ epiblast; _hy._ primary hypoblast; _bp._ Van Beneden's so-called blastopore. The shading of the epiblast and hypoblast is diagrammatic.] In the formation of the embryo out of the embryonic area the phenomena which distinguish the Sauropsida from the Ichthyopsida are repeated. The embryo lies in the centre of the area; and before it is formed there appears a primitive streak, from which there grows out the greater part of the mesoblast. At the front end of the primitive streak the hypoblast and epiblast become continuous, though a perforated neurenteric blastopore has not yet been detected. All these Sauropsidan features are so obvious that they need not be insisted on further. The embryonic evidence of the common origin of Mammalia and Sauropsida, both as concerns the formation of the layers and of the embryonic membranes, is as clear as it can be. The only difficulty about the early development of Mammalia is presented by the epibolic gastrula and the formation of the blastodermic vesicle (figs. 178 and 179). That the segmentation is a complete one is no doubt a direct consequence of the reduction of the food-yolk, but the growth of the epiblast cells round the hypoblast and the final enclosure of the latter, which I have spoken of as giving rise to the epibolic gastrula, are not so easily explained. [FIG. 179. RABBIT'S OVUM BETWEEN 70-90 HOURS AFTER IMPREGNATION. (After E. van Beneden.) _bv._ cavity of blastodermic vesicle (yolk-sack); _ep._ epiblast; _hy._ primitive hypoblast; _Zp._ mucous envelope.] It might have been supposed that this process was equivalent to the growth of the blastoderm round the yolk in the Sauropsida, but then the blastopore ought to be situated at the pole of the egg opposite to the embryonic area, while, according to Van Beneden, the embryonic area corresponds approximately to the blastopore. Van Beneden regards the Mammalian blastopore as equivalent to that in the Amphibia, but if the position previously adopted about the primitive streak is to be maintained, Van Beneden's view must be abandoned. No satisfactory phylogenetic explanation of the Mammalian gastrula by epibole has in my opinion as yet been offered.