THE FORMATION OF THE ECTODERM AND ENTODERM 



41 



edges of the lip on each side are carried caudad and brought together. 

 Thus the crescent is transformed into a longitudinal slit, as in Fig. 26. 

 The lips of the slit fuse, and the line of fusion is marked by the longi- 

 tudinal primitive groove. This 

 interpretation of the primitive 

 groove and streak, shown in 

 Fig. 22, is known as the con- 

 crescence theory. According to 

 the theory, the crescentic groove 

 of reptiles and birds is homo- 

 logous with the blastopore of 

 Amphioxus. As it is trans- 

 formed into the primitive streak 

 and groove, these represent a 

 modified blastopore. Accord- 

 ing to this view, a large part 

 of the entoderm of birds, rep- 

 tiles, and mammals is formed 

 by gastrulation, as in Amphi- 

 oxus. 



Gastrulation in Mammals. — As in reptiles and birds so also in mammals, 

 a process resembling gastrulation takes place but after the formation of the yolk 

 entoderm. A primitive streak appears at the posterior border of the germinal 



Fig. 23. — The primitive streak of pig embryos (Kei- 

 bel). A, embryo with primitive streak and primitive 

 node; B, a later embryo in which the medullary groove is 

 also present, cephalad in position. 



Post, openinq of notochord canal 

 ~^x / Primitive streak 



Ant.opentny of /Int. persisting portion of 



notochord canal notochordal canal 



Fig. 24. — Median longitudinal section through the blastoderm of a bat (Ycspcriilio murinus) 



(after Van Beneden). 



area (Fig. 23), with a crescentic opacity corresponding to the crescentic groove 

 of birds. Longitudinal sections (Fig. 24) of the germinal area of the bat show the 

 formation of a dorsal or notochordal plate which has replaced, and is fused later- 

 ally with, the yolk entoderm. A blastopore or notochordal canal is present lead- 



