ACCOUNT OF THE GERM-CELL CYCLE 49 
and thus the diploid or somatic number of chromo- 
somes is regained. 
When the spermatozoén enters an egg which has 
completed polar-body formation, the head does not 
Fic. 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 2. B, polar bodies formed; approach of the nuclei. C, union 
of the nuclei. D, approach of the nuclei. £, 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. 
E 
