324 CARNEGIE INSTITUTION OF WASHINGTON. 



inheritance of differences in size and its explanation; (3) the action of 

 selection in changing inherited characters. 



Five brief papers have been published and one comprehensive paper, 

 PubHcation No, 288. 



My former assistant, Mr. L. C. Dunn, has returned from overseas 

 service and has resumed his interrupted investigations. 



Morgan, T. H., Columbia University, New York. Study of the constitution 

 of the germ-plasm in relation to heredity. (For previous reports see Year 

 Books Nos. 15-17.) 



The following report covers the work of T. H. Morgan, C. B. Bridges, 

 and A, H. Sturtevant for the year 1919 on the constitution of the 

 hereditary materials of the pomace fly, Drosophila melanogaster. 



The working up of the data concerning the localization of the genes 

 of the third chromosome has occupied much of our attention. New 

 characters have continued to appear, the gene for one of which has 

 increased the length of the third-chromosome map by about one quar- 

 ter of its former known length. The recurrence of previously known 

 mutations is being carefully watched and checked, because of their 

 bearing on many fundamental problems, such, for instance, as the 

 relative mutability of different loci, the determination of a minimum 

 figure for the number of loci in the germ-plasm of Drosophila, and their 

 bearing on evolutionary problems (such as the increased chance of a 

 beneficial mutation becoming established, parallel evolution, etc.). 



Attempts to cross D. melanogaster with other species have been 

 persistently made, but unsuccessfully so far, with one exception. The 

 causes for this failure, when mating is known to occur, are being fur- 

 ther investigated, and new species, when brought into the laboratory, 

 are tried out in all possible combinations. It is hoped to continue this 

 work in other parts of the world where untried species abound. As a 

 prehminary to this sort of work, Sturtevant's taxonomic study of all 

 North American species of this group is nearing completion and will 

 soon be submitted for publication. 



It has been found possible to obtain hybrids between D. melano- 

 gaster and an undescribed species. A yellow mutant in the new species 

 (found by Dr. Metz) furnished an opportunity for a direct test of the 

 supposed identity of mutant characters in the two species. It has been 

 crossed with yellow melanogaster and has given yellow hybrids. Both 

 yellows being recessive, it follows that they are in the same locus. This 

 is the first time that parallel mutations in different species have been 

 proven to be identical, though other methods have made this more or 

 less probable in several cases. Even more important in this connection 

 is the peculiar sex ratio in these hybrids and their sterility. Both of 

 these problems are under investigation by Mr. A. M. Brown and others. 



Further study of non-disjunction is being carried out on material 

 more favorable than any other hitherto available even in Drosophila — 



