162 



Heredity and Environment 



FIG. 56. DIAGRAMS OF SEX DETERMINATION IN MAN. A, Spermato- 

 gonium with 47 chromosomes one of which (small circle) is the X 

 chromosome. B, Spermatocyte showing 23 synaptic pairs and a single 

 unpaired X chromosome. C, Reduction division in which the synaptic pairs 

 separate while the X chromosome does not divide, consequently the sec- 

 ond spermatocytes D and D' contain respectively 23 -f- X and 23 chromo- 

 somes. E and ', Second maturation division in which every chromo- 

 some divides, giving rise (F) to two equal classes of spermatids and 

 spermatozoa, one of which has 24 chromosomes and the other 23. If an 

 egg containing 24 chromosomes is fertilized by a sperm with 24, a female 

 with 48 chromosomes is produced; if an egg with 24 chromosomes is 

 fertilized by a sperm with 23, a male with 47 chromosomes results. 

 (After Morgan.) 



A similar condition in which one race has twice as many chro- 

 mosomes as another race of the same species is found in two 

 races of the thread worm, Ascaris megalocephala. Still more 

 recent work by Wieman shows that the number of chromosomes 

 in white men and in negroes is 24; in the male there is presum- 

 ably an XY pair, in the female an XX pair. 



On the other hand, Evans* found in hundreds of counts that 



* See Babcock and Clausen, p. 538. 



