63 



impregnation the germinal vesicle disappears and the germ- 

 yelk (PI. I. fig. 4, a) contracts. This constant effect of im- 

 pregnation indicates the presence of an attractive force which 

 has produced a certain condensation of the germ-yelk. If 

 the yelk-membrane (b) were previously in contact with the 

 chorion (c), it has now receded from it, and an albuminous 

 fluid (d) occupies the interspace. 



The primary germ-cell (e) soon appears at or near the 

 centre of the germ -yelk. Its nucleus divides (fig. 5), and 

 two germ-cells result ; these recede and establish two cen- 

 tres of attraction and assimilative force, around which, in 

 many species, the germ-yelk collects, and is thus divided 

 (fig. 6). Each secondary germ-cell becomes two (fig. 7)> 

 and the matter of the germ-yelk is collected around four 

 centres (fig. 8). The same process again takes place in 

 each of these four, producing an eight-fold division, then 

 a sixteen-fold one (fig. 9), and by its rapid repetition the 

 germ-yelk becomes subdivided and assimilated into count- 

 less germ-cells (fig. 10), and by their further subdivision 

 the nuclear or granular germ-mass (fig. 11) is established. 

 This is the most common mode in which the primary im- 

 pregnated germ-cell propagates itself at the expense of the 

 germ-yelk. But in some species the germ-yelk is not at- 

 tracted round the successively developed centres, but the 

 germ -cells as they are propagated push their way through 

 the germ-yelk and progressively assimilate it: the ulti- 

 mate result being the same a germ-mass of countless 

 secondary germ-cells or nuclei. Much extraneous matter 

 has been assimilated and combined with the essential mat- 

 ter of the primary impregnated germ-cell in order to form 

 the material out of which the embryo is to be developed : 

 but something more is done besides increase of bulk by 

 assimilation and division. 



The reception of the matter or principle of the sperma- 

 tozoon by the germinal vesicle is essential to the com- 

 mencement of this growth by multiplication of cells. When 



