ACCOUNT OF THE GERM-CELL CYCLE 49 



and thus the diploid or somatic number of chromo- 

 somes is regained. 



When the spermatozoon enters an egg which has 

 completed polar-body formation, the head does not 



F 



FIG. 11. Diagrams of two principal types of fertilization. I. Polar 

 bodies formed after the entrance of the spermatozoa (annelids, 

 mollusks, flat-worms). II. Polar bodies formed before entrance 

 (echinoderms). 



A, sperm-nucleus and centrosome at $ ; first polar body forming 

 at 9- B, polar bodies formed; approach of the nuclei. C, union 

 of the nuclei. D, approach of the nuclei. E, union of the nuclei. 

 F, cleavage-nucleus. (After Wilson.) 



have time to transform into a nucleus as large as 

 the egg nucleus, but nevertheless fuses with the latter 

 (Fig. 1 1, D, E 9 F) . Although the two nuclei are very 

 unequal in size, they possess an equal amount of 

 chromatin and furnish an equal number of chromo- 

 somes to the first cleavage spindle. 



