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 



Fig. 11. — Diagrams of two principal types of fertilization. I. Polar 

 bodies formed after the entrance of the spermatozoa (annelids, 

 moUusks, 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. 11, D, E, 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. 



