THE CELLULAR BASIS 121 



ter cells where they form the daughter nuclei. 

 Each of these cells therefore receives half of 

 its chromosomes from the egg and half from 

 the sperm. Even in cases where the individual 

 chromosomes are lost to view in the daughter 

 nuclei those nuclei may sometimes be clearly 

 double, one-half of each having come from the 

 egg chromosomes and the other half from the 

 sperm chromosomes (Fig. 26). 



At every subsequent cleavage of the egg 

 the chromosomes divide in exactly the same 

 way as has been described for the first cleav- 

 age. Every cell of the developing animal re- 

 ceives one-half of its chromosomes from the 

 egg and the other half from the sperm, and 

 if the chromosomes of the egg differ in shape 

 or in size from those of the sperm, as is some- 

 times the case when different races or species 

 are crossed, these two groups of chromosomes 

 may still be distinguished at advanced stages 

 of development. Where the egg and sperm 

 chromosomes are not thus distinguishable it 

 may still be possible to recognize the half of 

 the nucleus which comes from the egg and the 

 half which comes from the sperm even up to 



