122 CARNEGIE INSTITUTION OF WASHINGTON. 



modification of a Mendelian unit. Dr. Metz has made progress in his 

 study of the evohition of the chromosome groups in flies and in the com- 

 parative factorial structures of chromosomes of related species. Dr. 

 Blakeslee is making genetical studies on a wide range of flowering plants. 



REPORTS ON INVESTIGATIONS. 



THE GERM-PLASM ANDIITS MODIFICATION. 



COMPARATIVE STUDY OF THE CHROMOSOME GROUPS IN DIPTERA. 



Dr. Metz reports as follows: 



"The chromosomes of Diptera, unlike those of most animals, are normally 

 arranged in symmetrical pairs, the two members of each pair remaining in 

 close association with one another during the greater part of the life of the fly. 

 Stages were studied all the way from the egg to the adult, and in each the same 

 paired association was found. During maturation these pairs undergo a 

 reduction division essentially like that in other insects (a fact previously 

 described by Stevens), and the two members go into different germ-cells. 

 It appears, then, that the egg and the sperm contribute equivalent sets of 

 chromosomes, and that in the fertiUzed egg corresponding members of the 

 two sets become associated in pairs and remain associated throughout the life 

 of the new individual. This is made extremely probable by the fact that in 

 many cases the chromosomes are of different sizes and shapes and may be indi- 

 vidually identified. In all cases (save X and Y in the male) the two members 

 of a pair are alike in size, form, and behavior. Indeed, in one species it is 

 possible easily to distinguish each individual pair from every other, and to see 

 that every chromosome associates with its corresponding mate. If the chromo- 

 somes are genetically continuous, as there is great reason to beheve, then these 

 facts leave no doubt that in the flies each chromosome from one parent pairs 

 up and remains associated with its mate from the other parent. 



"A careful study of these phenomena — especially the evidence from tetra- 

 ploid groups, as given in the paper — lends considerable support to the hypothe- 

 sis that pairing is the result of a constitutional (chemico-physical) similarity or 

 likeness between corresponding members, i. e., each chromosome seems to have 

 a definite make-up, similar to that of its mate, but different from that of the 

 others; hence it associates with its particular partner and with no other. 



"Obviously these results lend material support to the theory that Mendelian 

 factors are located in the chromosomes — a theory necessitating just such a 

 definite qualitative structure and individuality as that which appears to exist 

 in these flies. 



"In connection with this investigation it became of considerable interest to 

 learn the distribution of the phenomenon of chromosome pairing; and for this 

 reason the study became much more comprehensive than it would otherwise 

 have been. It involved about 80 species, from among the highest to the 

 lowest families of Diptera. The chromosome behavior was found to be the 

 same in all of them, leading to the conclusion that the paired association is 

 characteristic of the order Diptera." 



COMPARATIVE STUDY OF CHROMOSOMES THROUGH THE GENUS DROSOPHILA. 



Dr. Metz has also undertaken a comparative study of the chromo- 

 somes in the genus Drosophila. In its entirety this study is a very 



