GENETICS OF HETEROMORPHIC CHROMOSOMES 459 



The first of these conditions has ah-eady been satisfactorily 

 met-; in a former paper (Carothers, '17), based on a study of the 

 germ cells of several species of a certain group of short-horned 

 grasshoppers, the following facts were established: 



1. Given tetrads (seven out of eleven in Trimerotropis fallax) 

 may be composed of morphologically dissimilar homologues. 



2. When heteromorphic, the members of these pairs segregate 

 during the first maturation division. This behavior parallels that 

 of unit characters as established by Mendel's first law. 



3. The homologues of three of these pairs were traced and 

 found to segregate at random in regard to the accessory chromo- 

 some and consequently in relation to each other, and presumably 

 to the members of other pairs, thus furnishing a physical mechan- 

 ism such as would be necessary for the carrying out of Mendel's 

 second law. 



4. The chromosomal constitution of approximately one hun- 

 dred wild individuals was such as would be expected from a free 

 union of gametes bearing these morphologically unlike homo- 

 logues. 



The logical conclusion was that here, with ahnost diagram- 

 matical clearness, we could trace the segregation of given homo- 

 logues to the gametes and their recombination in the zygotes. 



At the same time, however, the alternative possibility was 

 pointed out that in these species there might be a reorganization 

 at the time of fertilization which would result in a shift in point of 

 fiber attachment and a corresponding change in the morphology 

 of the chromosomes, such that the offspring of a single pair 

 would tend to give the range of variation of the species instead 

 of the range possible from a combination of the gametes of their 

 parents. The possibility that there might be a reorganization 

 was rendered more probable since at about the same time McClung 

 ('17) encountered relations between the octad and hexad multiples 

 of Hesperotettix viridis which seemed to indicate a reorganiza- 

 tion of the chromosomal combination involved in the multiples 

 at the time of fertilization. The two cases, as Dr. McClung 

 pointed out, are not exactly comparable, since in H. viridis non- 

 homologous chromosomes are involved and the mechanism may 



