THE MORPHOLOGY OF FERTILIZATION 9 



brane becomes known as the fertilization membrane, which is 

 about 500 A thick (Mitchison, 1953). As will be seen from an 

 examination of Fig. 3, the cortical granules which disappear at 



^i^arrr?:;■:{■^;^^>.~^^■■;^:v■^*-;u■•^''■'^'vt??S?''V 



(a) imsn^§Mm 



:CKXiaO£H>Q. 



■g 



(b) l:^:^ff^fp}p^^W§M 



(c) 



(d) 



(e) 



- ^ ^ 



-/ 

 -h 





FIG. 3. — Formation of the fertilization membrane in the sea-urchin egg, after 

 Endo (1952). a. Unfertilized egg; b, extrusion of cortical granules; c, ad- 

 hesion of cortical granules to vitelline membrane; d, further transformation 

 of fertilization membrane ; e, completely transformed fertiUzation membrane. 

 V, vitelline membrane; /), plasma membrane; g, cortical granules; h, hyaline 

 layer;/, fertilization membrane. Note. Diffraction effects at the surface of a 

 large egg make it extremely difficult to distinguish by optical methods 

 closely apposed layers which are less than 1-2 n thick. 



fertilization in fact fuse with the inner surface of the vitelline 

 membrane, a phenomenon which was first systematically examined 

 by Motomura (1936, 1941), though Just observed the escape of 

 granules from the cortex, their appearance in the perivitelline 

 space, and possibly their incorporation into the fertilization 



