112 CARNEGIE INSTITUTION OF WASHINGTON. 



indication of resemblance between the sex-linked group of characters in 

 willistoni and any of the groups in melanogaster , so it will be necessary to 

 secure many more mutants before this feature can be cleared up." 



The conclusion that can be drawn from the foregoing data is that 



the cytological resemblance of 

 chromosome groups may not safely 

 be taken as a basis for conclusions 

 concerning genetical relationships. 

 It follows, also, that the tracing of 

 T^ „ . " ' , ^ T^ .,,' . - chromosome evolution in the genus 



D. melanogaster 9 D. wdlistom 9. i , , , ? 



T^_ o T^- r u , Drosophila will probably be found 



riG. z. — iJiagrams of chromosome-complex. ..^j, . 



Sex chromosomes are shown in solid black. more dirhcult than waS at OnO 



time hoped. 

 The breeding experiments with Drosophila willistoni (carried on 

 largely by Mrs. Lancefield) have included the analysis of 27 sex-linked 

 characters and a smaller number of non-sex-linked characters The 

 data on the sex-linked characters have been prepared for publication. 

 In addition to the special points mentioned above, they indicate that 

 the phenomena of crossing over in willistoni are similar in general to 

 those in melanogaster. The genes for the 27 characters fall into a 

 inear series, based on cross-over percentages, giving a "map" approxi- 

 mately 86 units long. 



Leria pectinata. 



Work with Leria pectinata has been started by Dr. Anderson and 

 the species has proved favorable for study. Isolations have been 

 made of new types appearing either as mutations or as segregates from 

 heterozygous flies found in the pigeon-houses. In a few cases the 

 "ofT-type" flies have been collected or reared from collected larvae. 

 Several mutant races have been established or are being isolated. 

 Some of these are of little value, because of low viability or great 

 variability of the characters. The present need is for more characters 

 that are sufficiently simple and clear-cut to be used as tools in the 

 analysis of the more complex and less clearly expressed ones. The 

 main objective of the present work is to accumulate as many of such as 

 possible and to standardize cultural conditions. 



EXPERIMENTAL MODIFICATION OF THE GERM-PLASM. 

 The Treatment of Mice with X-Rays Before Mating. 



Dr. C. C. Little, in collaboration with Dr. Halsey T. Bagg, research 

 fellow in biology of the Huntington Fund for Cancer Research, has 

 been studying the efifects of exposing mice to very small doses of 

 X-rays. The experiment was begun in the autumn of 1920. A 

 closely inbred strain of colored mice was used and the treatments were 

 made before mating. Five consecutive daily treatments were given 



