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, ata 
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. 
