THE MORPHOLOGY OF FERTILIZATION 19 



fertilization, such changes always resulting in the egg becoming 

 more spherical. For example, the unfertilized egg of Urechis caupo 

 has a large indentation in its surface, which disappears a few 

 minutes after fertilization, the egg becoming spherical, Plate II 



23 



31 



32 



32'A 



32 y^ 



35 '/s 



36 



43 



58 



65 



FIG. 6. — Changes in the shape of the egg oi Ascidiella aspersa (O. F. Miiller) after 

 fertihzation (external membranes removed), after Cohen & Berrill (1936). 

 The numbers below each drawing are minutes after fertilization. 



(Tyler, 1932). The eggs of Cumingia tellitioides (Conrad) are not 

 spherical before fertilization, become transiently ovoid thirty 

 seconds after fertilization and round up in about a minute (Morgan 

 & Tyler, 1930), while every student knows that some batches of 

 sea-urchin eggs are pear-shaped before, and completely spherical 



