THE ORIGIN OF GYNANDROMORPHS. 11 



It is a curious fact that practically all of the mosaics of Drosophila 

 involve the sex chromosomes. It is true that the differences in the 

 sexes are so marked that individuals partly male, partly female, could 

 easily be detected on this basis alone. On the other hand, the mutant 

 characters that are sex-linked are not more striking than are those of 

 autosomal mutants. The almost complete absence of the latter kind 

 of mosaics in our cultures shows very positively that elimination is very 

 infrequent in these chromosomes, or, if it occurs, that an individual or 

 part with only one autosome is less likely to survive than an individual 

 with one X chromosome. Until this question is settled it can not 

 safely be concluded that the sex chromosomes suffer elimination more 

 than do the autosomes. The fact that autosomal non-disjunction has 

 not yet been observed in Drosophila, though looked for, lends support 

 to the view that variations in autosomal number are either rare or are 

 fatal. 



RELATIVE FREQUENCY OF ELIMINATION OF THE MATERNAL 

 AND PATERNAL SEX CHROMOSOME. 



It might have been supposed a priori that delay in the unraveling of 

 the chromosomes of the sperm might be the most frequent cause of 

 the elimination of chromosomes. As a matter of fact, the evidence 

 shows clearly that the maternal X is as likely to be eliminated as the 

 paternal. For example, we find on looking through our records that in 

 15 cases the maternal X chromosome and in 15 cases the paternal 

 chromosome must have been the one eliminated. There were 16 cases 

 hi which from the nature of the cross or of the result it could not be 

 determined which one was eliminated. In the above estimation we 

 also have left out of account all cases that were entirely male, or for 

 which special explanations are called for. There can then be no doubt 

 but that elimination is somehow connected with the nature of the X 

 chromosomes themselves, such as slowness in dividing or in reaching the 

 poles of the spindle, and that elimination is not due to delay in the 

 development of either pronucleus. 



An examination of the gonads in Drosophila gynandromorphs has 

 shown hi every case that the two gonads are the same, i. e., both are 

 ovaries or both are testes. Even in bilateral types the two gonads are 

 alike. Duncan found this true for the few cases that he sectioned. 

 This number was, however, insufficient to establish the rule, but we can 

 now add about 20 other cases to the list. There can remain no doubt 

 that the gonads are alike, regardless of the way in which the male and 

 female parts are distributed on the surface. The results are in accord 

 with the early formation of the germ-cells in Diptera and probably mean 

 that both gonads are derived from one and the same cleavage nucleus. 



