222 THE PERPETUATION OF ADAPTATIONS 



accord with the description given. For example, the bug 

 Protenor 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 di- 



d.etertni/taticn in titan ( 



\M*. 



< 



^ 



'? c f jfc 



atx" *?|V;j 



B 







FIG. 99. Diagram of the history of the male cells in human spermatogenesis. 

 A, Spermatogonium with forty-seven chromosomes; B, first spermatocyte with 

 the haploid number of chromosomes in pairs and the sex chromosome (open 

 circle); C, first maturation division; D, two resulting cells (spermatocytes) from 

 the first maturation division; E, division of the second spermatocytes giving 

 F, four resulting spermatozoa, two female producing (above), two male producing 

 (below). (From Morgan.) 



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

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

 vision 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, 9 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- 



