550 



AMPHIBIA. 



the presence of yolk acts as a check on the activity of the pro- 

 toplasm, we can understand why the smaller cells continue to 

 divide much more rapidly than the large yolk-containing cells, 

 and so how the smaller epiblastic cells gradually spread over 

 the egg, covering in the larger ones. At one point, where 

 upper and lower cells meet, a groove is formed. According 

 to the older view, at this point the small cells are invaginated, 

 and so form a cavity ; according to recent research, the 

 cavity is simply formed by the splitting of the large cells. 

 However this may be, the cavity, which is the archenteron 

 or embryonic gut, rapidly enlarges at the expense of the 

 segmentation cavity, which soon disappears. The groove 

 becomes a circular aperture in the epiblast, which has now 



FIG. 185. Division of Frog's Ovum. (After ECKER.) 

 The numbers indicate the number of cells or blastomeres. 



spread over the whole egg except at this spot, the blastopore. 

 The embryo elongates slightly, but the mass of yolk-laden 

 cells which lie on the floor of the gut prevents the body 

 acquiring at once the fish-like shape. The blastopore as 

 usual marks the posterior region of the body. 



The processes which follow are already in outline familiar 

 to the student. Along the mid dorsal line an epiblastic 

 neural plate is differentiated. The edges become raised 

 into the neural folds, these approach one another and, fusing 

 together, form the medullary or neural canal. At the 

 posterior end this communicates with the archenteron for a 

 time by the neurenteric canal. Internally, a differentiation 



