GERM LAYERS. 



91 



prominent and the opening at its anterior end, corresponding to Hensen's node, 

 is usually well marked. The line of the head process is strongly marked by a 

 deep groove the neural groove (compare Fig. 84 with Fig. 54, a). 



A longitudinal section in the medial line of this disk (Fig. 85) shows a re- 

 markable similarity to a corresponding section of the bat's disk (Fig. 55). The 

 ectoderm consists of a single layer of columnar cells interrupted only at the 

 opening of the blast opore (anterior end of the primitive groove). The entoderm 

 (chorda anlage) also consists of a single layer of cells which is continuous at the 

 blastopore with the ectoderm. In the region of the primitive groove the per- 



Yolk sac 



Amnion 



Neural groove * 



Chorion - 



V^ <^f 



FIG. 84. Dorsal view of human embryo, two millimeters in length, with yolk sac. 



von Spec, Kollmann. 

 The amnion is opened dorsally. 



istomal mesoderm is present. The embryonic disk forms the roof, so to speak, 

 of the yolk sac. 



A transverse section (Fig. 86) through the primitive groove shows all three 

 germ layers fused in the medial line, but separated laterally. In this case there 

 is a striking resemblance to the condition seen in a corresponding section of the 

 rabbit's disk (Fig. 87). 



Apart from the embryonic disk, the conditions are very similar to those in 

 Peters' ovum (compare Figs. 85 and 82). 



The unusual feature in both these embrvos is the enormous extent of the 



