45° 



ZOOLOGY FOR MEDICAL STUDENTS 



on the egg becomes resolved into a large number of cells arranged in a 

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

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



Fig. 1S7. 



Segmentation of the egg of Ampkioxus. (After Hatschek.) The second polar body is seen 

 adhering to the siu'face of the egg in the neighbourhood of the apical pole. 



those towards the apical pole are distinctly smaller than those towards 

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

 character — the granules consisting of minute particles of reserve food- 

 material or yolk. 



There now ensues the process of gastrulation by which the blastula 

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



