224 THE PERPETUATION OF ADAPTATIONS 



ceiving seven chromosomes (Fig. 98, 9 ,D, E,). Now if a sperma- 

 tozoon with six chromosomes fertilizes one of these eggs the 

 result is a male with thirteen chromosomes; if one with seven 

 chromosomes fertilizes the egg, the result is a female with four- 

 teen chromosomes. The large, odd, chromosome therefore is a 

 sex determining chromosome. 



Another excellent illustration is given by the nematode worm 

 Ancyrocanthus where the number of chromosomes may be 

 counted in the living germ cells. The male, as in Protenor, 

 produces two kinds of spermatozoa, one with five chromosomes, 

 the other with six. The eggs all contain six chromosomes. 

 Fertilization with one type of spermatozoa produces a male 

 with eleven chromosomes; fertilization with the other type 

 produces a female organism with twelve. 



In man there is some evidence that a similar difference in 

 spermatozoa is present, although the small size of the chromo- 

 somes and their large number makes counting difficult, so that 

 observers disagree as to the facts. According to one careful 

 observer (von Winiwarter) the male cells contain forty -seven 

 chromosomes which unite to form twenty-three pairs and one 

 odd chromosome (Fig. 99). Two types of spermatozoa result, 

 one with twenty-four the other with twenty -three chromosomes. 

 Female cells have forty -eight chromosomes according to this 

 observer's best counts, twenty -four being present in the mature 

 egg. Fertilization results in an embryonic cell with forty-eight 

 or forty-seven chromosomes according to the type of sperma- 

 tozoon uniting with the egg cell, and the resultant individual 

 is female or male according to the type of spermatozoon. 



The cytological evidence therefore affords some very clear 

 proofs that in some cases at least sex varies with the presence 

 or absence of one chromosome and we cannot get away from the 

 conclusion that in such cases this chromosome itself is the 

 essential factor in sex determination. In other cases the evi- 

 dence is equally clear but it is found that the sex chromosome is 

 not always alone, i.e., unpaired at maturation. In some cases 

 its history may be easily followed because of some size difference 

 or other peculiarity. Thus in the bug Lygaeus the sex chromo- 



