218 SUPPLEMENT. 



or the cell-membrane of the yelk-cell, closely encompasses the 

 germ-vesicle at first and then gradually expands, while between 

 it and the vesicle a transparent fluid collects ; in which, at a 

 later period, a turbidness commences, occurring first in the 

 neighbourhood of the germ-vesicle. Wagner had thus dis- 

 covered in the course of his observations that the details of 

 the process were just what must have been expected according 

 to the theory of the unity of the principle of development for 

 all elementary particles of the organism. That the germ- 

 vesicle is the nucleus of the yelk-cell appears to me therefore 

 to be scarcely dubitable. The illustration given by Wagner 

 also shows that the germinal spot is first developed, then the 

 germ-vesicle around it, and around this again the yelk-cell. It 

 is not surprising that granulous contents may form within 

 the germ-vesicle at a subsequent period, since the same thing 

 occurs in the indubitable nucleus of the adipose cells of the 

 fish, and the formation of the cell is probably nothing more 

 than a repetition of that same process around the nucleus, by 

 means of which the nucleus was originally formed around the 

 nucleolus. 





