CHROMOSOME STUDIES IN THE DIPTERA 55 



another and to the first that the individual chromosomes can be 

 homologized throughout the entire series, as they can be in the 

 Drosophilas. 



Pairing and conjugation of chromosomes 



The Drosophilas offer some of the most striking evidences of 

 the actual pairing of chromosomes in the diploid groups, thus far 

 observed. The phenomenon is apparently characteristic of all 

 Diptera,^ but is nowhere so striking as in this genus. 



The figures speak for themselves in this regard, and need little 

 emphasis. The sex-chromosomes alone show any appreciable 

 number of exceptions to such a paired association. A number of 

 cases have been found in which these were not paired, and in 

 general the association between them is less intimate than that 

 between members of autosome pairs. Whether this is correlated 

 with the precocious .contraction of the sex-chromosomes, or is due 

 to an inherent difference between them and the autosomes, cannot 

 be determined. In any event, the difference is slight, for nor- 

 mally they are paired like the autosomes. 



In such cases as these, where two or three pairs of chromosomes 

 can be individually identified, there can be no question that each 

 pair is composed of one maternal and one paternal component, since 

 any germ cell of either sex contains only one member of each of 

 these kinds. And when it is evident that three out of five pairs 

 are so made up (e.g.. Type IV), a very strong presumption is 

 created that the other two pairs are similarly constituted. 



But the most remarkable feature of the whole study is the dis- 

 covery that the chromosomes not only exhibit a close association 

 in pairs at nearly all times, but that before every cell division the 

 members of each pair become so intimately united that they may 

 be said actually to conjugate. Each pair, with the possible ex- 

 ception of the sex-chromosomes, goes through what amounts to 

 a synapsis in every cell generation, so that in many cases the 



° Miss Stevens ('08, '11) records it in nine species of Muscidae, and four species 

 of mosquitoes. I have verified it in five of these species of Muscidae, and extended 

 it to eight others, in addition to those of Drosophila. 



