128 CARNEGIE INSTITUTION OF WASHINGTON. 



"In addition to cytological work on the genus Drosophila, the chromosomes 

 of about 20 species, representing various families of the Diptera, have been 

 studied, with a view to learning more of the characteristics of the order as a 

 whole. When the preparations made during the summer are studied, many 

 more species will be added to the above and extensive material will be available 

 for a comparative study of several families. Some features already appear 

 to be clear. First, the paired association of chromosomes is characteristic of 

 the Diptera as a whole. If any species lack it they are the exceptions. None 

 such has yd been found. Second, several families of flies (Muscidse, Sarco- 

 phagida?, Syrphidae, Anthomyidae, Scatophagidae, etc.) all appear to have the 

 same number of chromosomes (12) and appear to be fairly uniform in their 

 maturation phenomena. Perhaps it may be said that they have reached a state 

 of equilibrium in these regards. The Drosophilas and their close relatives, on 

 the other hand, exhibit great specific differences, both in chromosome numbers 

 and in maturation processes. As a group they do not seem to have become 

 stable. 



"Since the phenomenon of chromosome pairing is so characteristic of the flies, 

 and found nowhere else in anything like such a definite form, I have made a 

 detailed study of it. This study is nearly completed and will appear as Xo. 

 II in my 'Chromosome Studies in the Diptera.' In this investigation I have 

 studied various species from a number of families, and have found the behavior 

 of the chromosomes, as regards pairing, to be essentially the same in all. I 

 have shown that pairing exists throughout the whole life of the insect, from 

 the embryo to the adult, and in the germ-cells previous to the maturation 

 stages. During each cell generation, somatic or germinal, the two chromo- 

 somes constituting a pair become closely associated, in fact almost fused, soon 

 after cell division; then later they separate somewhat and lie side by side until 

 the next cell division. In other words, the maternal and paternal chromosomes 

 of a pair are associated with one another throughout the entire life of the 

 animal, and once during each cell generation they become very closely united. 

 These facts, of course, strongly suggest that the two members of a pair are 

 qualitatively alike and that the members of difTerent pairs are qualitatively 

 unlike. 



"The breeding work in which I have been engaged is of several difTerent sorts. 

 Primarily it has centered around the endeavor to cross different species of 

 Drosophila, which I have demonstrated to possess different chromosome 

 groups. In jireparation for this work it has been necessary, first of all, to 

 perfect methods for rearing the various species, which in itself has taken much 

 time, because the food habits of some are very different from those of others. 

 Nevertheless over 15 species have been bred successfully, and methods have 

 been secured which will probably suffice for almost any species subsequently 

 obtained. With cultural methods perfected another attempt is now being 

 made to secure species hybrids. 



"In addition to keeping up stock for hybridization work, four species are being 

 extensively bred for genetic work purelj^ within species. Four species have 

 been selected, each representing a different tj'pe of chromosome group, and it 

 is hoped that mutants will be secured, making it possible to test the chromosome 

 hypothesis by breeding from them. If good series of mutations are secured 

 in these 4 species, or in 2 or 3 of them, I will have a crucial test of the chromo- 

 some theory from the genetic side, for each species should give a definite 

 result, characteristically different from the others. The work on D. ampelo- 

 phila has shown the presence in that species of 4 groups of linked characters, 

 corresponding to the 4 pairs of chromosomes. 1 have selected one species 

 having this same group of chromosomes. From it I should get results like 



