454 SUMMARY OF CURRENT RESEARCHES RELATING TO 



heavier. 2. The mothers in good condition have heavier offspring. 

 3. The heavier mothers have heavier offspring, for they are older and 

 in better condition. 4. Individuals in small litters weigh more at birth 

 than do individuals in large litters. 5. The weight increases in later 

 litters, probably because the mothers are older. 6. It is probable that 

 the prolongation of the gestation period for even one day materially 

 increases the weight of the young at birth. 



Influence of Temperature on Development of a Mendelian 

 Character.* — Mildred A. Hoge has found that selection aimed at pro- 

 ducing races of Drosophila possessing high or low numbers of teeth in 

 the sex-comb resulted in the isolation of races with high and low 

 numbers of teeth ; but in neither race was the number of teeth of the 

 average individual higher or lower than the extreme variants in a wild 

 stock. 



During the course of the selection a mutation, involving reduplica- 

 tions in the legs, appeared. The 'origin of the mutation was probably 

 not determined in any way by the selection, for repeated selection in a 

 new line was not followed by a similar character. The new character 

 was found to be due to a sex-linked factor, the location of which in the 

 sex-chromosome is close to that of the factor for vermilion eyes. 



The extra legs sometimes acted as a dominant, and sometimes as a 

 recessive character, and flies homozygous for the reduplicating factor 

 were often perfectly normal. The reduplications were found to be due 

 to some extent to the temperature, as a low temperature, maintained 

 throughout the larval life, was necessary for the production of a large 

 proportion of abnormal flies. Only flies carrying the reduplicating 

 factor, however, could be thus affected by a lowering of the temperature. 



The extra legs were of a variety of types, and the number of extra 

 parts in a single leg varied from one to four. A definite relation in 

 symmetry was found to exist between the normal and the supernumerary 

 parts. It is apparent that the extra parts are formed by one or more 

 bifurcations, and it is suggested that reduplications in other animals 

 may be explained in a similar manner. 



Ovarian Factor in Recurrence of (Estrous Cycle. f — F. H. A. 

 Marshall and J. Gr. Eunciman find that the occurrence of " heat " 

 (pro-oestrum and oestrus) in dogs does not depend upon the presence of 

 mature or nearly mature Graafian follicles in the ovaries. It is equally 

 evident that it is not dependent upon corpora lutea. 



It must be supposed, therefore, that the ovarian factor in the recur- 

 rence of "heat" resides in some other ovarian element or combination 

 of elements. The ovarian interstitial cells are possibly concerned in the 

 process, but cyclical changes in the condition of these cells have not so 

 far been observed in the dog's ovaries. 



In any case, the view which has generally been maintained, that the 

 ripening of the Graafian follicles and the onset of menstruation or of 

 heat stand to one another in the relation of cause to effect, must be 



* Journ. Exper. Zool., xviii. (1915) pp. 241-96 (5 pis.). 

 t Journ. Phys., xlix. (1914) pp. 17-22 (2 figs.). 



