496 HEREDITY AND SEX 



determination of these two kinds of egg-cells begins very early 

 in embryonic life, and is completed at maturation. 



W. E. Castle has proposed an ingenious hypothesis, which also 

 assumes two kinds of eggs and two kinds of spermatozoa, but 

 is distinctive in the further hypothesis that " male " spermatozoa 

 can only fertilise " female " eggs, and that " female " spermatozoa 

 can only fertilise " male " eggs. Thus the fertilised e^g repre- 

 sents a union of "male" and "female" gametes, and the 

 determination of the sex is left undetermined. 



Two Kinds of Spermatozoa. — In about thirty different kinds 

 of animals, such as the fresh-water snail Paludina, there are two 

 distinct forms of spermatozoa, and it has been suggested that the 

 two kinds are respectively predisposed towards the two sexes. 

 But there is no definite evidence of this. 



More interesting is the fact, first noticed by Henking in 1890 

 in the bug Pyrrhocoris, and corroborated in various other insects 

 by a number of American zoologists, that half the spermatozoa 

 differ from their neighbours in having an extra chromosome. 

 McClung has suggested that the presence or absence of this 

 accessory chromosome is correlated with the sex of the offspring. 

 Spermatozoa with the accessory chromosome are supposed to 

 tend to produce males. Further inquiry, however, has shown 

 (for some cases) that the female organism has in its body-ceUs 

 one more chromosome than the male has, and it seems almost 

 certain that the eggs have one more chromosome than half of the 

 spermatozoa have, and the same number of chromosomes as 

 the other half of the spermatozoa have. When a spermatozoon 

 with the same number of chromosomes as the ovum fertihses 

 an ovum, the result is supposed to be a female. When a sperma- 

 tozoon with one chromosome less than the ovum fertilises an 

 ovum, the result is supposed to be a male. It is possible to 

 conceive that the accessory chromosome is the vehicle of the 

 primary constituents of femaleness, or that the mere quantitative 

 preponderance due to an extra chromosome gives the fertiUsed 



