176 Papers from the Marine Biological Laboratory at Tortugas. 



character; meaning, according to the general interpretation, that hornless- 

 ness is determined by the presence of some determiner which prevents the 

 development of horns. Similarly, femaleness may be due to the presence 

 of the X-element, preventing the development of maleness.^ It would be 

 immaterial whether it were contributed by the male or by the female gamete. 

 This idea is further suggested by, and seems in perfect accord with, the 

 numerous observations indicating that femaleness is undeveloped maleness, 

 e. g., embryological facts; relatively female characteristics of male infants; 

 the assumption of male secondary characters after spaying, or after the 

 menopause; etc. 



SUMMARY AND CONCLUSIONS. 



Among the forms examined, including mongoose, cat, squirrel, pig, 

 rabbit, white mouse, sheep, horse, mule, bull, and dog, heterochromosomes 

 are apparently lacking in the male germ-cells of the first five, and present in 

 the remainder. The available evidence favors more the interpretation in 

 terms of a bipartite univalent or compound X-element than of an asso- 

 ciated X and Y group (idiochromosomes). 



In view of the fact that heterochromosomes have recently been reported 

 in man (Guyer, double X-element; Gutherz, equal pair of idiochromosomes, 

 or X- and Y-elements), rat (Guyer), armadillo (Newman and Patterson), 

 guinea-pig (Stevens), and opossum and bat (Jordan), the evidence indicating 

 similar elements in the above-enumerated group of six common mammals 

 would seem to warrant the conclusion that sex-chromosomes are very 

 generally present in mammals. Universality of presence seems vitiated for 

 the present by the fact that in another group of five mammals, carefully 

 studied, such elements seem apparently lacking. It might be assumed 

 that such elements are actually present in the male germ-cells, but are so 

 small or labile as to elude detection by ordinary methods, or do not present 

 the usual morphology of heterochromosomes during the prophase stages. 

 The unmistakable presence, however, of a "split-accessory" in the female 

 germ-cells (primary oocyte) of the cat, as recorded by Winiwarter and Sain- 

 mont, and the absence of any X-element in the male (confirmed by Gutherz, 

 1912), suggests very forcibly that sex-chromosomes are present in all 

 mammals, generally in the male, exceptionally in the female. The same 

 result would follow (that of numerical sex-equality) whether present in one 

 or the other sex. If this hypothesis can be further sustained, it would seem 

 cogently to reinforce the evidence for an essential sex-determining function of 

 heterochromosomes. Interpreted in terms of Mendelian heredity-formulae, 

 in those mammals in which an X-element is present in the male, the female 

 sex is homozygous, the male heterozygous. The facts would seem to fit the 

 hypothesis that the accessory chromosome acts as a deterrent to the develop- 



' However, in the case of horn-heredity, the inhibitor, according to this explanation, is dominant; in sex- 

 heredity, recessive. In cases where the female is the heterozygote, the condition might be parallel to that of 

 horn-heredity. In these instances the female would be simplex, the male nulliplex. Recession of a duplex 

 condition seems a contradiction in terms. The suggestion can have significance in only an approximate or 

 general sense. 



