132 CARNEGIE INSTITUTION OF WASHINGTON. 



finement (by closing the front door) when a compartment is wrongly chosen and 

 by reward of food (which is revealed by raising the back door) when the right 

 compartment is entered. The right compartment is never the same one 

 in successive trials, but always bears a certain relation to the other open front 

 doors. There may be some question as to whether a rat can have such an 

 idea as the first door to the right (this being the relation of the correct door in 

 the first problem), yet it is easily seen that in mastering such a problem the 

 activity of the rats offers considerable insight into their relative mental capaci- 

 ties. As far as possible, all the rats trained with the puzzle-box will be further 

 used in this experiment on the effects of alcohol. An additional reason for 

 employing the multiple-choice method is to provide a check on the results of 

 the puzzle-box training. Agreement in the two sets of results would give a 

 basis for generalizations; disagreement would leave doubt as to the validity 

 of either method. The technique of training has been developed, and at 

 present twenty rats are being tested for intelligence by this apparatus." 



A second investigation that has been planned is on the effect of tem- 

 perature and humidity upon the germ-plasm. To make this experiment 

 possible an extensive air-conditioning apparatus was installed in the 

 animal house by the Harry Bentz Engineering Company. A detailed 

 description of it is given toward the end of this report. Dr. MacDowell 

 assumed charge of the experiments planned for this apparatus. About 

 20 rats were raised from infancy under the various combinations of con- 

 ditions afforded by the air-conditioning apparatus, viz, hot dry, hot 

 moist, cold dry, and cold moist. From these animals 10 controlled 

 matings were made. Just when the next generation appeared it became 

 necessary, through lack of funds, to discontinue the operation of the 

 refrigerating machine. It is highly desirable to resume the experiment, 

 even though it is expensive. 



THE SIGNIFICANCE AND CONTROL OF SEX. 



\Miile the view that a difference in the sex chromosomes is sufficient 

 to account for differences in sex characters is widely accepted by 

 biologists, yet evidence is accumulating that the chromosomal differ- 

 ence is merely one difTerential and that there are probably others. Dr. 

 Riddle has continued to collect evidence that supports the conclusion 

 that an important differential associated with sex is the storage metab- 

 olism of the egg (Year Book, No. 13, pp. 117-119). Riddle's work during 

 the present year confirms and amplifies his earlier conclusions. 



As is well known, certain entomostracans of the group Cladocera are 

 ordinarily parthenogenetic and for long periods females only are 

 produced. Recently Keilhack, after going over all of the evidence, 

 concluded that, while Simocephalus may behave differently, all species 

 of the genus Daphnia and many other species of Cladocera which he 

 names can not reproduce parthenogenetically for as long a period as six 

 months. Unfavorable conditions may, indeed, hasten sexual repro- 

 duction, but constant conditions can not postpone the production of 



