I. THE ORIGIN OF GYMNDROMORPHS. 



By T. H, Morgan and C. B. Bridges. 



INTRODUCTION AND GENERAL DISCUSSION. 



The sharp distinction into two kinds of individuals, males and 

 females, characteristic of so many animals, is occasionally done away 

 with when an individual appears that bears the structures peculiar to 

 the male in some parts and to the female in other parts of the body. 

 Such an individual may show not only the secondary sexual differences 

 (either sex-limited or sex-linked) of male and female, but gonads and 

 genitalia of both kinds as well. We speak of these as gynandromorphs. 

 The union of the two sexes in a single individual shows how far the 

 characteristics normally associated with one sex alone are compatible 

 with the presence in another part of the same body of somatic structures 

 and reproductive organs of the opposite sex. In a word, how far each 

 is independent of sex hormones. But the chief importance of these 

 rare combinations lies in the opportunity they furnish for analysis of 

 the changes in the hereditary mechanism of sex determination that 

 makes such combinations possible. This evidence is chiefly derived 

 from gynandromorphs that are also hybrids. Such individuals may 

 combine not only male and female sex differences, but the character- 

 istic racial differences as well. Whether gynandromorphs arise more 

 frequently in hybrids or whether it is only that their detection is easier 

 under such circumstances will be discussed later. The occurrence of 

 hybrid gynandromorphs offers at any rate a unique opportunity to 

 discover the method of origin of such kinds of individuals. 



In hybrid gynandromorphs the differences that are shown may be 

 due to genes carried by the sex chromosomes. Most of the gynandro- 

 morphs of Drosophila belong to this category. In many cases, how- 

 ever, especially in other insects, it is not known whether the differences 

 shown by the hybrid gynandromorph are due to the sex chromosomes 

 or to other chromosomes, either because the ancestry of the gynandro- 

 morph is unknown or because the method of inheritance of the gene 

 is unknown. There are, however, some very rare cases in Drosophila 

 in which the characters involved are probably autosomal and the 

 individual, while showing its dual parentage in different parts of the 

 body, is not a sex-mosaic. It may be convenient to designate such 

 types as mosaics, while the sex-mosaics may be designated by the 

 more special term gynandromorphs. 



In our work on Drosophila melanogaster (ampelophila) a large number 

 of gynandromorphs and mosaics have appeared, and since the first 



