GROWTH AND REPRODUCTION 49 



appears in the production of two kinds of eggs, 

 large eggs yielding females and small eggs yielding 

 males. More usually the sexual predestination is 

 seen not in the egg cells but in the male germ cells, 

 or spermatozoa. The male animal possesses two 

 kinds of spermatozoa, equal in number. One kind 

 makes males, the other kind makes females. In this 

 case the difference between the two kinds of sperm 

 cells goes hand in hand with a highly remarkable 

 difference in the nuclear substance of these cells. 

 The sperm cells which make females have one or 

 more chromosomes in excess of the number pos- 

 sessed by those sperm cells that make males. Where 

 the female-producing sperms have one more chromo- 

 some than the 'male-producing sperms, the extra 

 chromosome is a peculiar one, often visibly dis- 

 tinguishable from the others in size and in certain 

 peculiarities of behavior. It is very noteworthy that, 

 owing to this peculiarity, the body cells of the female 

 always have one more chromosome than the male. 

 Similarly, it has been found that where the sperms 

 yielding females have two or three more chromosomes 

 than the sperms [yielding males, there is a corre- 

 sponding excess of chromosomes in the body cells 

 of the females. 



In the phylloxerans and aphids every fertilized 

 egg makes a female. The explanation of this extraor- 

 dinary fact has been furnished by the experimental 

 biologist Morgan. It is that in these cases the male- 

 producing spermatozoa are rudimentary and with- 



