48 GERM-CELL CYCLE IN ANIMALS 
contents which result in the formation of a uninuclear 
cell, the zygote. As a rule one spermatozoén 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 spermatozoén may occur 
before, during, or after the polar body formation 
(Fig. 11). If the spermatozoén enters before the 
maturation of the egg is completed (A), 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 spermatozoén and 
that of the mature egg approach each other and 
~ecome 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 
