OOGENESIS 



121 



the spermatozoon, surrounded by a small cell- body consisting 

 of cytoplasm with centrosphere and centrosome, enters the 

 ovum. It moves towards the egg-nucleus, and its two 

 chromosomata become resolved into a tangled skein of 

 chromatin. The chromosomata of the egg-nucleus, after the 



Fig. 25. 



Maturation of the ovum in Ascaris ineglocephala var. bivalens. A, the ovum, 

 with the spermatozoon, Spz, entering it. The nucleus contains two 

 tetrads. B, the nucleus has approached the surface and the tetrads are 

 dividing. C, the nucleus has divided to form pb' ', the first polar body ; 

 both egg-nucleus and polar body contain two dyads. D, the dyads 

 in the egg-nucleus rotating preparatory to a second division. E, division 

 of the dyads in the egg-nucleus and in the first polar body pb' . F, for- 

 mation of the second polar body and division of the first polar body. There 

 are three small cells (the polar bodies), and one large cell (the ovum), 

 each containing two chromosomes. The spermatozoon has entered the 

 ovum, and its nucleus, covered by a granular cap of archoplasm, is shown at 

 pn (J . (Adapted from Boveri.) 



formation of the second polar body, follow suit, and the 

 centrosome and attraction sphere of the egg-nucleus seem 

 to disintegrate and disappear altogether. As the sperm- 

 nucleus approaches the egg-nucleus, the centrosome of the 



