572 



GENERATION. 



[CHAP. XL. 



character, and were produced by the active vibration of exceed- 

 ingly delicate cilia, which had become developed upon the surface 

 of the yolk. This interesting movement may be observed in the 

 ova of frogs and other animals. It can always be seen in those of 

 the common water-snail (Limnceus stagnalis), and forms a most 

 interesting object for observation. 



Cleavage of the Yolk. Soon after the ovum has become im- 

 pregnated, and the germinal vesicle has disappeared, the yolk 

 of the mammalian animal divides into two large cells, and 

 each of these again subdivides into two, so that the yolk mass 

 now consists of four cells, which soon become sixteen in number ; 

 these sixteen, thirty-two ; and so on, until at last the yolk consists 

 of an aggregation of spherical cell-like bodies. Each one of these, in 

 the mammalian ovum, is about the 1 -2000th of an inch in diameter, 

 and consists of a central light portion, and an external dark part. 

 The external part, which is composed entirely of yolk granules, is 

 not covered by an investing membrane, but the yolk granules 

 seem to be simply aggregated round the central clear vesicle, 

 which, when set free, looks very like an oil-globule. According to 

 Bischoff, this vesicle cannot be looked upon in the light of a nucle- 

 ated embryonic cell. It is probably more correctly considered as 

 a nucleus, around which yoke granules have become attracted. 



The process of cleavage of the yolk is very easily observed in the 

 eggs of the batrachia (frogs, newts, etc.), in which animals the 

 change may be watched from day to day. The division of the 

 yolk has been also very carefully investigated in the ova of certain 

 entozoa, in different species of which two distinct modes of divis 

 have been demonstrated. 



Kolliker found in the ova of certain species of Ascaris (niyrovenosa, 

 minata, and succisa), that after the disappearance of the germinal vesicle, 

 new nucleated cell appeared in the centre of the yolk, and in a short til 

 two such cells manifested themselves. Round each one of these cells poi 

 tions of yolk-mass collected, so that soon the entire yolk was divided inl 



Fig. 269. 



Cleavage of the yolk in Ascaris nigrovenosa and acuminata, shewing successive steps of the 

 process of yolk-cleavage. The three figures to the left are after Kolliker, the two others after 

 Bagge. 



