226 THE PERPETUATION OF ADAPTATIONS 



to the number of chromosomes in development of the sperm 

 cells of certain animals. In a great many insects (belonging 

 to the orders hemiptera, diptera, homoptera, phylloxerans, 

 etc.), in nematode worms (Ascaris, Ancyrocanthus) , and in 

 Guinea pigs, the process of spermatogenesis does not exactly 

 accord with the description given. For example, the bug 

 Pro tenor has thirteen chromosomes in the cells of the male, and 

 fourteen in those of the female, instead of the same number in 

 both sexes. Of the thirteen male chromosomes, one is consider- 

 ably larger than the others. At synapsis the smaller chromo- 

 somes unite in pairs according to the usual rule, but the large 

 one remains unpaired (Fig. 98, ^ A). At the first maturation 

 division, all of the chromosomes divide, six small and one large 

 passing into each daughter cell. At the second maturation divi- 

 sion, the six small ones divide again, while the large one passes 

 undivided into one of the daughter cells (Fig. 98, ^ D). Thus 

 two types of spermatozoa result, one type possessing six chro- 

 mosomes, the other, seven. 



In the female germ cells (Fig. 98, $ A, B), there are fourteen 

 chromosomes, of which twelve are smaller than the other two. 

 The latter unite in synapsis and behave like the smaller chromo- 

 somes during maturation divisions, the resultant eggs all re- 

 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 my 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 pnale 

 with eleven chromosomes ; fertilization with the othert^^ pro- 

 duces a female organism with twelve. 



In man, there is some evidence that a similar difference in 



