xn 



DEVELOPMENT 



201 



the white until only a small circular space the blastopore, 

 filled by a mass of yolk-cells the yolk-phtg, is left 

 uncovered (G, H, yk. pi}. 



Development of the Chief Organs In the mean- 

 time a second cavity appears in the interior of the 

 embryo, and as it increases in size, the segmentation- 

 cavity undergoes a proportional diminution and finally 

 disappears. This new cavity, or enteron(Figs. 64, 1, and 65 



ent 



mcs 



d 



c 



FIG. 65. Transverse sections through the body of a Frog embryo. A. with open 

 medullary groove (compare Fig. 64, J). B, with closed medullary groove 



ent), is the rudiment of the enteric canal : it begins to be 

 formed as a narrow slit at the edge of the blastopore 

 through which it at first communicates with the exterioal 

 but it soon becomes closed : at present there is neither 

 mouth nor "anus. 



By this time the cells of which the embryo is com- 

 posed have assumed different forms and become arranged 

 in a very definite manner. First of all there are severa 

 layers of small pigmented cells, derived from the black 



