70 



TEXT-BOOK OF EMBRYOLOGY. 



streak, its anterior club-shaped end to Hensen's node. If we assume the same 

 transformation of the crescentic groove into the primitive groove, the two to- 

 gether corresponding to the blastopore, the condition is quite analogous to that 

 in the chick (p. 65). 



At a slightly later stage than shown in Fig. 53, a new opacity appears ex- 

 tending forward in the medial line from Hensen's node (Fig. 54, a). This is 

 the head process, and may be considered as homologous with the head process in 

 the chick. (Compare Fig. 54, a, with Fig. 50.) The opacity is due to a plate 

 or cord of cells which grows from the region of Hensen's node forward under the 

 surface layer of cells (ectoderm) (Fig. 55). On the assumption that Hensen's 



Embryonic disk — 



Hensen's node 



Primitive 

 streak 





■- P V ( 



-ft. 



■ Si 



■ s 2 



s 3 



I 



Peristomal 

 mesoderm 



w . .■- 



F IG - S3- — Embryonic disk of dog. Bonnet. The letters and figures on the right (S1-S4) indicate 



planes of sections shown in Fig. 75. 



node is the anterior lip of the blastopore, this plate of cells may possibly be con- 

 sidered as homologous with the invaginated cells which form the protentoderm 

 in Reptiles and Birds. (Compare Figs. 42, 51 and 55.) Consequently, since 

 the protentoderm in the lower forms was designated the "primitive intestinal 

 cord" (Urdarmstrang), so in Mammals this invaginated cord of cells maybe 

 called the "primitive intestinal cord" (protentoderm) (Fig. 54). 



In Reptiles it has been seen that as the protentoderm grows forward under 

 the surface layer (ectoderm) the yolk entoderm for some distance disappears, 

 and the protentoderm fuses with the remaining yolk entoderm in an area 

 known as the completion plate (Fig. 42). In the chick also it has been stated 

 that a similar process occurs (p. 66). In Mammals the yolk entoderm, which 



