II. I 



CELL-DIVISION 



33 



here, it is evident, other factors must come into play, in the second 

 case probably a bilateral symmetry in the constitution of the 

 cytoplasm. These exceptions may, however, ultimately prove to 

 be special cases of Hertwig's rule. 



A very striking confirmation of the rule is to be found in the 

 division of the egg of Ascaris nigrovenosa (Figs. 17, 18). The 



FIG. 17. Four stages in the fertilization of the egg of Ascaris 

 nigrovenosa. (After Auerbach, from Korschelt and Heicler.) 



egg of this worm is ellipsoid. At one end (that turned towards 

 the upper end of the ovary) the polar bodies are extruded, and here 

 the female pronucleus is placed. The spermatozoon enters at the 



FIG. .18. Three diagrams of the rotation of the fertilization spindle in 

 the egg of Ascaris nigrovenosa. e, s, the directions in which the female 

 and male pronuclei approached one another in A ; 1, 2, 3, successive 

 positions of the spindle. (From Korschelt and Heider, after 0. Hertwig.) 



1 



JENKINSON 



