At the edge of the area pellucida the hypoblast becomes continuous with a thickened rim of material, underlying the epiblast, and derived from the original thickened edge of the blastoderm and the subjacent yolk. It is mainly formed of yolk granules, with a varying number of cells and nuclei imbedded in it. It is known as the germinal wall, and is spoken of more in detail on pp. 160 and 161. [FIG. 93. DIAGRAMS ILLUSTRATING THE POSITION OF THE BLASTOPORE, AND THE RELATION OF THE EMBRYO TO THE YOLK IN VARIOUS MEROBLASTIC VERTEBRATE OVA. A. Type of Frog. B. Elasmobranch type. C. Amniotic Vertebrate. _mg._ medullary plate; _ne._ neurenteric canal; _bl._ portion of blastopore adjoining the neurenteric canal. In B this part of the blastopore is formed by the edges of the blastoderm meeting and forming a linear streak behind the embryo; and in C it forms the structure known as the primitive streak. _yk._ part of yolk not yet enclosed by the blastoderm.] The changes which next take place result in the complete differentiation of the embryonic layers, a process which is intimately connected with the formation of the structure known as the primitive streak. The meaning of the latter structure, and its relation to the embryo, can only be understood by comparison with the development of the forms already considered. The most striking peculiarity in the first formation of the embryo Bird, as also in that of the embryos of all Amniota, consists in the fact that _they do not occupy a position at the edge of the blastoderm, but are placed near its centre_. Behind the embryo there is however a peculiar structure--the primitive streak above mentioned--which is a linear body placed in the posterior region of the blastoderm. This body, the nature of which will be more fully explained in the chapter on the comparative development of Vertebrates, is really a rudimentary part of the blastopore, of the same nature as the linear streak behind the embryo in Elasmobranchii formed by the concrescence of the edges of the blastoderm (_vide_ p. 64); although there is no ontogenetic process in the Amniota, like the concrescence in Elasmobranchii. The relations of the blastopore in Elasmobranchii and Aves is shewn in figs. B and C of the diagram (fig. 93). [FIG. 94. AREA PELLUCIDA OF A VERY YOUNG BLASTODERM OF A CHICK, SHEWING THE PRIMITIVE STREAK AT ITS FIRST APPEARANCE. _pr.s._ primitive streak; _ap._ area pellucida; _a.op._ area opaca.] [FIG. 95. TRANSVERSE SECTION THROUGH A BLASTODERM OF ABOUT THE AGE REPRESENTED IN FIG. 94, SHEWING THE FIRST DIFFERENTIATION OF THE PRIMITIVE STREAK. The section passes through about the middle of the primitive streak. _pvs._ primitive streak; _ep._ epiblast; _hy._ hypoblast; _yk._ yolk of the germinal wall.]