FERTILIZATION 



175 



the breaking down of these vacuoles and the discharge of their 

 substance from the surface of the ovum (Figs. 89, 90). This 

 substance may be in part transformed into, or may carry 

 before it a modified surface layer of material which appears then 





FIG. 90. Fertilization in the egg of Amphioxus. C, after Cerfontaine, others 

 after Sobotta. A. Ovarian egg showing cortical plasm. B. Cortical layer form- 

 ing a membrane on the surface of the egg, within the vitelline membrane. C. 

 Egg membrane fully formed but still attached to surface of egg. D. Extruded, 

 fertilized egg. Membrane fully formed and beginning to leave the surface of 

 the egg. c. Cortical layer; e, endoplasm; m, egg membrane; externally vitelline, 

 internally a product of the exoplasm; p, perivitelline space; s, spermatozoon; 

 v, vitelline membrane; /, first polar body; II, second polar spindle. 



as & fertilization membrane; this may be the vitelline membrane 

 or it may be an addition to a previously present vitelline 

 membrane (Fig. 90). Then either by the shrinkage of the egg, 

 or the expansion of the membrane, or both, or by the rapid ab- 



