98 CARNEGIE INSTITUTION OF WASHINGTON. 



willistoni) and the chromosome maps (based on amount of crossing over) 

 correspond roughly to the chromosomes in size, except in the case of one 

 map in D. virilis. In this species there are 5 long chromosomes and 1 very 

 short one, and there should be a corresponding series of crossover maps; but 

 instead the data thus far obtained give 4 long and 2 very short maps, which 

 suggests that crossing over may be reduced in one of the 5 large chromosomes. 



" One of the two hnkage groups showing little or no crossing over included 

 the characters bent and net, which resemble the characters bent and dimin- 

 ished in D. melanogaster. The latter have been shown by Bridges to be due 

 to genes in the small chromosome of D. melanogaster. This suggests a homol- 

 ogy between the small chromosomes of the two species, but we have not yet 

 been able to make sure of the point, because, with two small linkage groups, 

 we can not tell which represents the small chromosome. Further studies 

 ought to settle this point soon, however. 



"The character bent in D. virilis involves several different parts of the 

 fly and is readily modified by environmental conditions. For this reason it 

 has been used in an attempt to learn something of the action of the gene 

 during development. In preliminary experiments we have been able to show 

 that the character reacts to low temperature at a definite time, just preceding 

 the pupation of the larva. Exposure to cold at other times has no effect. We 

 have also found that different parts of the fly appear to differ from one another 

 in the time at which they are affected (within the limits just mentioned), 

 which seems to indicate that the gene acts successively in different regions, 

 or else that it sets up reactions which give that effect." 



Other Diptera. 



The cytological studies on chromosome behavior, chromosome relation- 

 ships, and gametogenesis, in other Diptera, particularly in the Drosophilinse, 

 have been continued during the year by Dr. Metz. 



EXPERIMENTAL MODIFICATION OF THE GERM-PLASM. 

 Effect of Cold in Inducing Datura Mutants. 



Since the last report, additional data have been secured on the effect o*f 

 cold in inducing mutations in Datura. No doubt the lowered temperature 

 induces abnormalities in the distribution of chromosomes during the forma- 

 tion of pollen and probably also during the formation of egg-cells. When the 

 new constant-temperature chamber is installed it should be possible to dis- 

 cover with greater precision the stage at which the stimulus should be applied 

 to bring the production of chromosomal mutations under a greater control 

 than has hitherto been possible. 



Genetics of X-rayed Mice. 



This investigation, begun by Drs. C. C. Little and H. J. Bagg in 1921 

 is being continued by Dr. Little at the University of Maine. He reports 

 that during the past year about 8,000 mice have been recorded. A lethal 

 head and jaw abnormality in mice (j) has been found to behave as a Mendelian 

 recessive. The eye abnormality (h) which occurred among the descendants 

 of the X-rayed mice has been found to be a Mendelian recessive, overlapping 

 normal. Abnormal feet and hair length in these X-rayed mice have proved 

 to be hereditary, and the exact method of inheritance is being investigated. 



