66 



TEXT-BOOK OF EMBRYOLOGY. 



of Hensen's node, there is a small area where fusion does not occur, thus leaving 

 a small opening which communicates with the cavity of the primitive gut. Since 

 the primitive groove is formed from the original crescentic slit, and the original 

 crescentic slit is the blastopore, the primitive groove may be considered as a 

 modified blastopore in which the only opening is at Hensen's node. The 

 primitive groove lies in the medial line of the primitive streak; and since the 

 primitive groove is a modified blastopore, the two primary germ layers are fused 



ec. en. arc. 



FIG. 48. From transverse section through Hensen's node germ disk of chick of 2 to 6 hours' incu- 

 bation. Diival. For lettering see FIG. 49. 



at the lips of the primitive groove (Figs. 48 and 49). To this fusion is due the 

 opacity which constitutes the primitive streak as seen from the surface (Fig. 46). 

 After the formation of the primitive groove and streak there is no longer any 

 specially marked definition of the posterior margin of the germ disk, the entire 

 circumference having a uniform demarcation. 



Very soon after the formation of the primitive streak a new opacity appears 

 which extends forward in the medial line from Hensen's node (anterior lip of 

 the blastopore). This is known as the head process, or "primitive intestinal 



en. ec. f.g. 





r ^-^^^s^^^^m^w^s^ ^5%>& 

 mi^^&^^^^f^^lMM^m^m 



FIG. 49. From transverse section through primitive groove germ disk of chick of 2 to 6 hours' incu- 

 bation. Duval. 



arc., Archenteron; ec., ectoderm; en., entoderm; l.b., lip of blastopore; p.g., primitive groove; y., 

 yolk; y.p., yolk plug. 



cord" (Bonnet) (Fig. 50). This new opacity is due to growth of cells under the 

 ectoderm, the cells constituting the protentoderm. As a matter of fact, this 

 formation of the protentoderm is a further extension of that same process 

 which began with the crescentic groove (blastopore) invagination and continued 

 during the transformation of the crescentic groove into the primitive streak 

 (still the blastopore). Consequently this whole process from the formation of 

 the crescentic groove up entirely through the formation of the protentoderm, is 



