48 GERM-CELL CYCLE IN ANIMALS 



contents which result in the formation of a uninuclear 

 cell, the zygote. As a rule one spermatozoon only 

 enters the egg (monospermy) ; but in a few species 

 (certain insects, selachians, tailed amphibians, reptiles, 

 and birds) many spermatozoa may normally fuse 

 with the egg (physiological polyspermy). The sper- 

 matozoon, which consists usually of three rather dis- 

 tinct parts, the head, the middle piece, and tail, 

 may become entirely embedded within the egg sub- 

 stance, or the tail may be left outside, or, in excep- 

 tional cases, only the head succeeds in entering. 



The union of the egg and spermatozoon may occur 

 before, during, or after the polar body formation 

 (Fig. 11). If the spermatozoon enters before the 

 maturation of the egg is completed (^4), its head 

 transforms into a nucleus equal in size to that of the 

 egg (C) ; the middle piece dissolves, giving rise to 

 a centrosome which inaugurates the formation of a 

 spindle with asters (B) ; and the tailpiece ap- 

 parently takes no active part in the fertilization 

 processes. The middle piece also does not seem to 

 be necessary for the formation of the centrosomes 

 and asters. The nucleus of the spermatozoon and 

 that of the mature egg approach each other and 

 come into contact between the asters (C). Then the 

 nuclear walls dissolve; a spireme which segments 

 into the haploid number of chromosomes is produced 

 by each nucleus, and the first cleavage spindle of 

 the developing egg results. This spindle bears the 

 haploid number of chromosomes from the spermato- 

 zoon and a like number from the egg nucleus 



