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 



