248 GERM-CELL CYCLE IN ANIMALS 
cleavage spindle, and thus the diploid (somatic) 
number is regained. These chromosomes may there- 
fore be considered as forming two groups, one group 
of paternal origin derived from the sperm nucleus, 
and one group of maternal origin derived from the 
egg nucleus; in fact the groups supplied by the two 
nuclei may remain perfectly distinct (Fig. 61), not 
only during the first cleavage division, but also 
during subsequent mitoses. 
The chromosomes of the fertilized egg and of 
the cells to which it gives rise are not always of the 
same size and shape, but in many cases are known to 
differ morphologically from one another. It is 
possible to recognize the different chromosomes 
during each mitosis, and the evidence is quite con- 
vincing that morphologically similar pairs are present 
in every cell and that one member of each pair is 
derived from the egg nucleus, the other from the 
sperm nucleus. Two principal views are held re- 
garding the character of the chromosome divisions 
during the early cleavage divisions, (1) that the 
chromatin granules, which represent definite de- 
terminers, are divided equally between the daughter 
chromosomes, and (2) that an unequal distribution 
of the granules occurs, thus forming daughter cells 
containing qualitatively different chromosomes. 
There are no observations which show an unequal dis- 
tribution. 
One of the changes that takes place in the chromo- 
somes at the time of mitosis is the diminution of their 
chromatin content brought about by the passage of 
