BIOLOGY. 277 



BIOLOGY. 



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, 16.) 



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

 Bridges, and A. H. Sturtevant for the year 1917-18 on the constitution 

 of the hereditary materials of the pomace fly, Drosophila melanogaster. 



Studies of the linear order of the genes in the chromosomes have con- 

 tinued to occupy the first place in our work, because Drosophila sur- 

 passes all other material for the analysis of this fundamental problem of 

 genetics. In our report for the first chromosome (Carnegie Inst. Wash 

 Pub. Xo. 237) the location of 33 genes was given. Twice as many more 

 are now known to us, the data for which will be ready for publication 

 before long. A report now in press (Carnegie Inst. Wash. Pub. No. 

 278) gives the data bearing on the location of 23 genes in the second 

 chromosome. An equal amount of data relative to as many more 

 mutant characters is being written up. The material needed for the 

 presentation of the third-chromosome data, including the description 

 of some 50 mutants, is also being prepared for publication. 



Complications have appeared in the localization problem, owing to 

 the discovery in Drosophila of specific variations in crossing-over caused 

 by definite cross-over genes (see especially Carnegie Inst. Wash Pub. 

 No. 278). Only a little has hitherto been published on this subject, al- 

 though we now know the effects of seven such genes in considerable 

 detail, besides having fragmentary evidence on others. Two of the 

 seven are in the first chromosome, three are in the second chromo- 

 some, and two are in the third. One of the third-chromosome cross- 

 over genes has, besides its effect on the third chromosome in which 

 it is situated, a very marked effect upon the second chromosome as well. 



There is in Drosophila an unique opportunity for the examination of 

 this important subject of linkage variation, on account of the com- 

 pleteness with which the normal relations are known and the advan- 

 tage of the closeness of analysis afforded by the very numerous mutant 

 loci. 



Aside from its intrinsic interest, the locahzation of a large number of 

 genes has furnished an opportunity for a successful analysis of the 

 classic problem of selection. A. H. Sturtevant has just brought 

 through the press the result of a year's work on this problem (Carnegie 

 Inst. Wash. Pub. No. 264). C. B. Bridges has sent to press a detailed 

 account of the specific modifiers of eosin (Jour. Exp. Zool.). T. H. 

 Morgan has in press a paper on genes that modify a dominant character 

 called ''notch" wing (Carnegie Inst. Wash. Pub. No. 278). 



During the last eight years many gynandromorphs of Drosophila have 

 appeared. This material has been brought together and a general 



