SPECIAL ARTICLES THE Y-CHROMOSOME IN MAMMALS THE majority of workers on mammalian spermatogenesis have described the sex-chro-mosome as being of the X-0 type but recent investigations in this field by the author in-dicate that the XY type of chromosomemay be more common than is generally thought. In the opossum,'an animal for which the X-0 type of sex-chromosome has been described, the writer finds a typical XY sex-chromosome complex. Both the X and Y com-ponents may be recognized in spermatogonial and somatic divisions because of their distinctive size. In the first maturation division the X and Y elements segregate apart to opposite poles of the cell, and in the second maturation division both divide equationally. Hence half of the sperm carry an X and half carry a Y chromosome. The diploid chromosome number for both the male and female opossum is 22, and not 17 or 24 as concluded by previous investi-gators.

In the testes of both the white man and the negro I have found in the first spermatocytes a chromosome pair which is similar in appearan (e and behavior to the XY chromosome of the opossum. The two members of this pair, in the human, representing the X and Y components, are unequal in size; they segregate apart in the first maturation division just as in the case of the oPOSsum. It will be of general interestto biologists to know that the diploid number of chromo-somes for man is very close to the number (47) given by Winiwarter. 2 In my own material the counts range from 45 to 48 apparent chromosomes, although in the clearest equatorial plates so far studied only 46 chro-mosomes have been found. Before a final conclusion is madeon the exact number it is desired to make a careful study of a large number of division plates. There can be absolutely no question, however, but that the diploid number of chromosomes for both the white man and the negro falls between 45 and 48. With the XY type of sex-chromosome we 1 The writer's work Row in press.