xvi 



PRODUCTION OF CELLS. 



cells, progressively increasing in number, which combine to form the first 

 material substratum of the tissues and organs of the embryo, and are hence 

 termed " embryonic cells." 



The outline of the process is this. The germinal vesicle disappears ; 

 the 3 7 elk shrinks somewhat together, then separates into two halves (B) ; 

 the first two segments divide each again into two (c), and the binary 



Fig. IX. DIAGRAMMATIC FIGURES TO ILLUSTRATE THK FORMATION OF CELLS WITHIN THE 

 MAMMALIAN OVUM BY SEGMENTATION OF THE YELK ; MAGNIFIED. 



a, external membrane ; b, yelk ; c, germinal vesicle containing the germinal spot ; d, 

 embryonic cells further magnified. 



division thus goes on (D, E,) pretty regularly, until a large group of small 

 segments is produced (r). These final segments are the embryonic cells (d) ; 

 they probably have no distinct, separable envelope ; but, like other naked 

 cells, they are formed of a nucleus and body of protoplasmic matter. The 

 nucleus is a globule of soft pellucid substance ; it may not be always dis- 

 coverable in the earlier segments, being hidden by the opaque granular 

 mass ; but it soon comes into view, and it seems to play an important part 

 in the formation of the cells. The nucleus in fact appears to initiate the 

 process of division, by itself dividing into two halves, which, speedily 

 acquiring full size, serve as two centres of attraction, round each of which 

 separately the yelk-substance is gathered, and thus parted into two new 

 segments. Possibly the substance of the vanishing germinal vesicle and 

 spot may give rise to the first nucleus. 



Fig. X. DIVISION OF THE YELK of ASCARIS. 

 o (from Kolliker), ovum of Ascaris nigrovenosa ; D and E, that of Ascaris 



A, B, 

 acuminata (from Bagge). 



