108 ORGANIZATION AND DEVELOPMENT OF ANIMALS IN GENERAL. 



For a long time the behaviour of the germinal vesicle at the 

 commencement of segmentation and its relation to the nuclei of the 

 first formed segments were obscure, and the knowledge of the changes 

 and fate of the spermatozoa which enter the ovum in the process of 

 fertilization was, in like manner, in a very unsatisfactory state. Of 

 late years, numerous investigations, especially those of Biitschli, 

 0. Hertwig, Fol, etc., have thrown some light on these hitherto 

 completely obscure processes. It was supposed that in a ripe ovum 

 preparing itself for segmentation the germinal vesicle disappeared, 



FIG. 102, a, 1. Parts of the ovum of Asterias glacialis with spermatozoa, embedded in 

 the mucilaginous coat (after H. Fol.) c, upper part of the ovum of Petromyzon (after 

 Calberla). Am, micropyle ; Sp, spermatozoa ; Jm, path of the spermatozoon ; Ek, female 

 pronucleus ; Eh, membrane of ovum ; Ehz, prominences of the same. 



and a new nucleus was formed quite independently of it ; and that the 

 persistence and the participation of the germinal vesicle in the for- 

 mation of the nuclei of the first segmentation spheres were exceptional 

 (Siphonophora, Entoconcha, etc.) Thorough investigations carried 

 out on the eggs of numerous animals have, however, shown that as 

 a matter of fact the germinal vesicle of the ripe ovum only experi- 

 ences changes in which the greater part of it, together with some of 



