228 THE PERPETUATION OF ADAPTATIONS 



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. 



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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.) 



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 



