45 



ZOOLOGY FOR MEDICAL STUDENTS 



.m tin- egg Uromes resolved into a large number of cells arranged in a 

 single layrr so as to form the wall of a spherical blastula (Fig. 187, I). 

 It will lu- noticed that these cells are not absolutely uniform in size: 



"-. * >- 



FIG. 187. 



the cKg of AmpliioxHS. (After llatsc.hek.) The second polar body is seen 

 to the surface of the egg in the neighbourhood of the apical pole. 



tho>e towards tin- apical pole are distinctly smaller than those towards 

 the opposite- pole. These latter possess cytoplasm of a more granular 

 ber -the granules consisting of minute particles of reserve food- 

 material or yolk. 



Tin -re now ensues the process of gastrulation by which the blastula 

 Averted into a gastrula (Fig. 188). This process is initiated 



