380 CARNEGIE INSTITUTION OF WASHINGTON. 



Eyeless, in stock, breeds nearer the normal type eye than in stock 

 selected from this for no eyes. The selected stock will, in time, pro- 

 duce a large number of flies that have no eyes at all. Here, again, 

 the evidence shows that modifying factors are involved. A selection 

 experiment with eyeless has been carried through 13 generations. 

 In one direction flies with the biggest eyes have been selected in each 

 generation, while in the other direction flies with the smallest eyes or 

 with none at all have been selected. The two stocks are now widely 

 different. It seems possible that a mutant allelomorph of eyeless arose, 

 in the stock selected downwards, that produces a more eyeless condi- 

 tion of the eyes, and probably this effect is enhanced by the addition 

 of modifiers. In the other direction the evidence indicates that modi- 

 fying factors alone are involved. 



Three new tumor stocks have been found. One is a lethal larval 

 tumor. In the other a tumor is present in the larva and is carried over 

 into the adult. Since the adults live and breed, this tumor is not 

 lethal. In fact, it has not been shown in the former case that the 

 lethal larva dies because of the presence of the tumor. On the con- 

 trary, it seems highly probable that the same agency that leads to the 

 development of the tumor is also the one that kills the larvse, but not 

 through the tumor. 



The question of the causes of mutation in Drosoyhila has always 

 interested us. Many experiments have been carried out in the 

 attempt to induce specific mutations by external agents, but since 

 no positive results have been obtained, we have refrained from pub- 

 lishing much work that has been done in this direction. At present 

 numerous attempts to call forth a Larger percentage of "reversion- 

 mutants" in the bar-eyed stock are being made. None so far has 

 given specific results. But, nevertheless, it may be of interest to record 

 that a series of experiments in which the eggs, larvse, and adults were 

 placed under very high atmospheric pressure gave no larger percentage 

 of return-mutation than is found in the bar stock under normal condi- 

 tions. At one time it seemed possible that by retarding the deposition 

 of the eggs for 10 days or more, a higher percentage of return-mutations 

 occurred, but on critical examination the result proved to be a non- 

 disjunction phenomenon. In stock that can not be so closely con- 

 trolled as Drosophila such a result might have led to a wrong conclu- 

 sion concerning mutation. Finally, a method has been devised by 

 which a related question can be tested, namely, whether when 

 mutation takes place it involves only one member of a pair of chro- 

 mosomes or both members at the same time. For the mutation prob- 

 lem an answer to this question is of great importance. The experiment 

 is still under way and should in time answer the question, but first it is 

 necessary to obtain a very large amount of data. 



