174 GENERAL EMBRYOLOGY 



Tprimary or a secondary process, this loss of water following 

 ' the entrance of the spermatozoon, appears as one of the 

 I important aspects of fertilization. 



In the eggs of many species there is a peripheral layer of 



FIG. 89. Changes in the structure of the ovum in Nereis, upon fertilization. 

 After Lillie. A. Unfertilized oocyte. Large germinal vesicle; cytoplasm con- 

 tains oil drops and yolk spheres, and shows well marked cortical layer (exoplasm). 

 B. Fifteen minutes after ensemination (the spermatozoon is not shown). The 

 cortical layer has chiefly gone to form a jelly-like layer outside the ovum, and is 

 not shown. C. Egg after centrifuging to show component substances, c, 

 cortical layer (exoplasm); n, germinal vesicle or nucleus; no, nucleolus; o, oil 

 drops; p, perivitelline space; v, vitelline membrane; y, yolk spheres; 1, layer of 

 oil drops; 2, hyaline cytoplasm with small basophile granules; 3, yolk spheres; 

 4, hyaline cytoplasm with large basophile granules. 



cytoplasm (exoplasm) which is comparatively clear, free from 

 granules, and characterized by the presence of fluid vacuoles 

 (Echinoderms, Nereis (Fig. 89), Amphioxus (Fig. 90), Teleosts; 

 see Chapter III). Entrance of the spermatozoon leads to 



