20 



Scientific Proceedings (50). 



14 (710) 



Heterochromosomes in mammals. 

 By H. E. Jordan. 



[From the Department of Anatomy, University of Virginia.] 



Heterochromosomes have now been reported for the male germ 

 cells of the following mammals: man and rat (Guyer, '10); 

 armadillo (Newman and Patterson, '10); opossum (Jordan, '11); 

 guinea-pig (Stevens, '11); and bat (Jordan, '12). Winiwarter and 

 Sainmont, '09, report a longitudinally split "monosome" in the 

 oocyte of the cat. 



A comparative study of mammalian spermatogenesis reveals 

 the absence of typical heterochromosomes in mongoose,* cat, 

 squirrel, rabbit and pig. Heterochromosomes are clearly present 

 at synapsis and prophase in the primary spermatocytes of the 

 following forms: white mouse, sheep, horse, mule, dog and bull. 

 Regarding dog, rabbit, and the monkey, the evidence is not yet 

 decisive. 



At certain stages the heterochromosomes (chromosome-nu- 

 cleoli) appear single (accessory; monosome), at others double or 

 bipartite. The latter appearance suggests a pair of idiochromo- 

 somes; but the body is more probably a split accessory. 



The absence of discernible heterochromosomes in the male, 

 and their conspicuous presence in the female, of the cat indicates 

 their presence in one or the opposite sex in all forms. If this 

 hypothesis can be supported by evidence from the oocytes of 

 mongoose, squirrel, pig, rabbit and similar forms, cogent additional 

 confirmation is given to the idea of a special significance of het- 

 erochromosomes, probably in connection with the determination 

 of sex. 



A simple explanation of sex-determination suggested by these 

 and other facts — and one in apparent accord with a large body of 

 experimental and cytological data — would seem to be to regard 



♦This material was collected at the temporary Marine Biological Station of the 

 Carnegie Institution of Washington at Montego Bay on the expedition to Jamaica, 

 B. W. I., in February and March, 1912, under the directorship of Dr. Alfred G. 

 Mayer. 



