302 



IXTRODFCTIOX TO CYTOLOGY 



confirm ( hiyer's report of a dimorphism among the sperms, though he also 

 observed the accessories. Guyer in 1914 reasserted his conclusions of 

 1912, but added thai he was finding a much larger chromosome number 

 in the cells of the white man. 



Yon Winiwarter (1912), also working upon the white man, found in 

 the spermatogonia and spermatocytes 47 chromosomes, including one 

 accessory. Since this accessory passes undivided to one pole in the first 

 mitosis and divides in the second, half of the sperms receive 23 and half 

 24 chromosomes. In the cells of the female there are 48. From these 

 data von Winiwarter logically concluded that the egg has 24 chromosomes, 

 and that it develops into a male when fertilized by a sperm with 23 

 chromosomes, and into a female when fertilized by one with 24. Such a 

 large number is found in the white man by Evans 1 also, but he finds 48 

 chromosomes rather than 47 in the spermatogonia, indicating the presence 

 of an XY pair as in Drosophila. 



M- 



Fig. 141. — Sex-chromosomes in man. A, primary spermatocyte, negro. B, same, 

 white. C, metaphase of heterotypic mitosis, negro. D, interkinesis, white. XY, the 

 sex-chromosomes; P, plasmosome. (After Wieman, 1917.) 



Wieman (1917), using both negro and white material, finds 24 chromo- 

 somes in the spermatogonia, two of them being distinguishable as an 

 unequal XY pair which remains condensed while the autosomes form the 

 reticulum (Figs. 140, 141). In the spermatocyte 12 pairs are evident, 

 including the XY pair. At the first maturation mitosis the 11 autosome 

 pairs separate into univalents as usual, but the X and Y divide longitud- 

 inally; thus each daughter cell (secondary spermatocyte) receives 11 auto- 

 somes and an XY pair. At the second mitosis the 11 autosomes divide 

 longitudinally in the normal fashion and the X and Y separate. As a 

 result all of the sperms receive 12 chromosomes : in half of them one of the 

 12 is the X and in the other half it is the Y. Although for a time it ap- 

 peared that the white man had twice as many chromosomes as the negro, 

 a difference ordinarily great enough to mark them as distinct species, 

 Wieman shows clearly that in his material the two have the same num- 

 ber, and is inclined to regard von Winiwarter's material as in some way 

 abnormal. Sex inheritance in man is evidently of the XY type, as 

 Wieman's researches and genetic data indicate with considerable clearness ; 

 but why some material should plainly show twice as many chromosomes 



1 Unpublished work cited by Babcock and Clausen, p. 538. 



