72 



DEVELOPMENT OF THE FROG'S EGG 



[Ch. VI 



embryo, we find that the laj-er 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 ventral! i/^ so as to nearly encircle this region of 

 the embryo. The blastopore is completely encircled by the 

 sheet of mesoderm. 



|^*^",_^''C^^ . ■ '■ "^ • ; ■ ■; '. ': V-'; 



D 



Fk;. '2(>. — A. Lonoitiiilinal section througli a yoimg embryo of Rana. B, C, D, 

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



Cross-sections through an older embr>"0 are drawn in Fig. 20, 

 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 embr3^o 

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

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



