THE CELLULAR BASIS OF HEREDITY 



A 



"5 



D 



H 



Fig. 25. Matdration and Fertilization of the Egg of the Mouse. A, first 

 polar body and second maturation spindle ; B, second polar body and maturation 

 spindle; C, entrance of the spermatozoon into the egg; D-G, successive stages in the 

 approach of egg and sperm nuclei ; H, formation of chromosomes in each germ nucleus ; 

 I, first cleavage spindle showing chromosomes from egg and sperm on opposite sides of 

 spindle. (After Sobotta.) 



grows thinner and finally disappears altogether, leaving the chromo- 

 somes in the equator of the spindle (Figs. 23 F, 24 E and F, 25 I). 



Each of the chromosomes then splits lengthwise into two equal 

 parts, and in the splitting of the chromosomes it is sometimes possible 



