136 



FERTILIZATION OF THE OVUM 



Allolobophoj-a (Foot), in the butterfly Picris (Henking), and in the 

 gasteropod Physa (Kostanecki and Wierzejski). The agreement 

 between forms so diverse is very strong evidence that this must be 

 regarded as the typical derivation of the centrosome.^ 



The facts may be illustrated by a brief description of the phe- 



Fig. 67. — Maturation and fertilization of tlie egg of the mouse. [SOKOTTA.] 



A. Tlie ovarian egg still surrounded by the follicle-cells and the membrane {:./., zona pel- 

 lucida) ; the polar spindle formed. B. Egg immediately after entrance of the spermatozoon 

 (sperm-nucleus at ,'^). C. The two germ-nuclei (j', ? ) still unequal; polar bodies above. 

 D. Germ-nuclei approaching, of equal size. E. The chromosomes forming. F. The minute 

 cleavage-spindle in the centre; on either side the paternal and maternal groups of chromosomes. 



nomena in the sea-urchin Toxopnc/istcs (Fig. 69). As described at 

 p. 146, the tail is in this case left outside, and only the head and 

 middle-piece enter the Qgg. Within a few minutes after its entrance, 

 and while still very near the periphery, the lance-shaped sperm-head, 

 carrying the middlcrpiece at its base, rotates through nearly or quite 



1 Cf. p. 156. 



