72 



DEVELOPMENT OF THE FROG'S EGG 



[Cn. VI 



embryo, we find that the layer of mesoderm is inserted be- 

 tween ectoderm and yolk-cells over all the posterior half of 

 the embryo. There is a small antero- ventral region into 

 which the mesoderm does not extend. At a point posterior to 

 the section described above, we find the mesoderm extending 

 much farther ventrally, so as to nearly encircle this region of 

 the embryo. The blastopore is completely encircled by the 

 sheet of mesoderm. 



Fig. 26. 



A. Longitudinal section through a young embryo of Rana. 

 E. Cross-sections of last in planes of lines in A. 



E 

 B, C, D, 



Cross-sections through an older embryo are drawn in Fig. 26, 

 B, C, D, E. The embryo has flattened along the mid-dorsal line. 

 The ectoderm has become thinner along this line, where a faint 

 groove can be seen on the surface of the living egg^ — the primi- 

 tive groove. On each side of the mid-dorsal line, the ectoderm 

 is somewhat thicker than before, and the cells are more closely 

 packed together. The ectoderm over the surface of the embryo 

 consists of an outer layer and of several inner layers of cells. 

 The cavity of the archenteron has opened out and is very large. 



