348 GENERAL EMBRYOLOGY 



the mesoderm itself is. After the separation of the chorda 

 and mesoderm, the endoderm proper, or enteroderm, as it is 

 called, is left as a thin narrow strip of cells spread flat over the 

 periblast (yolk) surface, continuous posteriorly with the diverg- 

 ing limbs of the germ ring. 



In the Sauropsids, where the accumulation of the yolk is most 

 pronounced, the blastoderm does not grow entirely around the 

 yolk until long after the gastrula is formed and the embryo 

 established. Correlatively we find no typical germ ring forma- 

 tion in the periphery of the blastoderm, save in that posterior 

 region which is to be concerned in embryo formation. Remem- 

 bering that in the Reptile both true invagination and enterocoel 

 formation occur, while these processes are not apparent in the 

 birds, we may describe (following Patterson's account) the 

 processes of gastrulation and embryo formation in the pigeon, 

 as illustrating these events in the development of the extremely 

 meroblastic ovum. 



The blastoderm first becomes quite thin, particularly toward 

 its posterior side, where, at the same time, the margin thickens 

 forming a segment of a true germ ring (Fig. 161). The exten- 

 sion of this posterior part of the germ ring, however, involves 

 the usual processes of cell multiplication accompanied by 

 involution and epiboly; there is no true invagination here (Fig. 

 161). The formation of an inner layer is thus limited to the 

 posterior region of the blastoderm. Soon, as this whole region 

 extends posteriorly, this segment of a germ ring begins to 

 contract toward the mid-line, and the result is the formation 

 of a median thickening in the posterior half or third of the germ 

 disc. This thickening is the primitive streak (Fig. 161), and as 

 usual it is the seat of the formation of the chief embryonic rudi- 

 ments. As in the Teleost, the primitive streak, formed by the 

 gradual fusion of the lateral halves of the germ ring, is obviously 

 the equivalent of the blastoporal margin of the frog or of 

 Amphioxus. On its surface is a shallow longitudinal groove 

 marking the separation of the two halves; this is the primitive 

 groove (Fig. 161), which may be regarded as representing the 

 blastopore proper. The archenteron, in such a gastrula as this 



