23O KATHARINE FOOT AND E. C. STROBELL. 



testes in the X-chromosomes and, as stated above, this would 

 effectually deprive the so-called male-producing spermatozoa of 

 an essential male-producing function. This is so evidently 

 out of harmony with the chromosome sex-determination theory, 

 that it needs no further comment. 



If we attempt to place the factors determining the testes in the 

 other sex-chromosome (the Y-chromosome) we meet difficulties 

 that are equally obvious, for there are many forms that have no 

 Y-chromosome at all. If we could ignore this important fact, 

 we would have, in these insects, quite a diagrammatic demon- 

 stration of the chromosome sex-determination theory, for the 

 Y-chromosome is the only chromosome that is in all the male- 

 producing spermatozoa, just as the X-chromosome is the only 

 chromosome that is in all the female-producing spermatozoa. 

 Each is the only chromosome which is distinctive of the type of 

 spermatozoa which it identifies. But the fact cannot be ignored 

 that the Y-chromosome, so conspicuous in these insects, is absent 

 in most forms, and we must therefore dismiss the possibility that 

 the factors determining the testes of these insects are carried by 

 this chromosome. The association therefore between the testes 

 and the sex-chromosomes can be no closer than we have shown 

 by experiment to be the case between the sex chromosomes and 

 the other two exclusively male characters the genital spot and 

 the intromittent organ. 



Realizing that the Y-chromosome cannot logically function as 

 the carrier of the factors determining a male, Morgan 'n sug- 

 gested that "the factors for producing the male must be in some 

 other chromosome" (than the Y- or the X-chromosomes). We 

 would consider this suggestion in relation to the factors deter- 

 mining the testes of these insects, ignoring for the present the 

 fact that in his diagram illustrating this suggestion, Morgan does 

 not place these factors in one chromosome but in a pair of chromo- 

 somes. If we attempt to place the factors for the testes in one 

 of the autosomes, we meet difficulties that are quite as obvious 

 as the difficulties in attempting to place the factors in the sex- 

 chromosomes. 



The above diagram (text Fig. i) shows that the spermatozoa 

 can be classed not only into two types (the so-called male-produc- 



