12f'» SUMMARY OF CURRENT RESEARCHES RELATING TO 



actively laying fowl is immediately inhibited by the injection in sus- 

 pension, in proper dosage, of the desiccated fat-free substance of the 

 corpus luteum of the cow. It has been shown by Loeb that one function 

 of the well-developed corpus luteum in the Mammalian reproductive 

 cycle is to inhibit ovulation. Its substance does the same in birds where 

 there is nothing corresponding to the corpus luteum. The duration of 

 this effect varies with different fowls from a few days up to two or three 

 weeks. After the hen begins ovulating again the laying goes on un- 

 impaired. The same effect is produced by the injection of extracts of 

 the lutear substance, either intravenously or intra-abdominally. The 

 active substance in producing the inhibition is inactivated by boiling. 

 According to the authors the fact that the inhibiting function of the 

 corpus luteum can be exercised in Birds as well as in Mammals, suggests 

 that natural selection had nothing to do with the evolution of either the 

 organ or the function in Mammals. But the argument does not appear 

 to us to be convincing on this point. 



Egg-production in the Domestic Fowl.* — R. Pearl and F. M. 

 Surface publish the third part of their biometrical study of egg-production. 

 The present paper deals with variation and correlation in the physical 

 characters of the egg, and presents quantitative data, with a biometrical 

 analysis of these, regarding the normal variation and correlation of the 

 egg of the domestic fowl in respect primarily of size and shape. The 

 material used was eggs of Barred Plymouth Pock pullets. The egg is 

 relatively more variable in length than in breadth, and more variable in 

 shape that in either of the linear dimensions, length or breadth. The 

 weight of the egg and its volume are more variable than any of the 

 other characters. There is a close agreement between the egg of the 

 domestic fowl and the human skull, in respect of the degree of variation 

 exhibited in the corresponding size and shape characters of the two 

 structures. A consideration of the processes concerned in the production 

 of the definitive size and shape of the egg and the skull suggests, though 

 it does not prove, that this similarity in regard to variation constants is 

 due to mechanical factors operative in both cases during the development 

 and fixation of the final form. With the exception of breadth, all 

 dimensional characters studied show significantly a symmetrical varia- 

 tion. All of the egg dimensions studied are correlated in varying 

 degree. Intra-racially egg length and egg breadth are correlated only 

 slightly. Neither the weight nor the volume of the egg is more than 

 slightly correlated with its shape. Both length and breadth are signifi- 

 cantly correlated with the weight and volume of the egg. The bulk- 

 measures (volume and weight) are more highly correlated with breadth 

 than with length. The specific gravity of the egg exhibits only a very 

 small degree of variation. The skull and egg show a correspondence in 

 respect to the degree in which their chief dimensions are correlated. A 

 comparison of the egg of the domestic fowl with those of a number of 

 species of wild birds leads to the result that in general the eggs of wild 

 birds are neither less nor more variable than those of the domestic fowl. 

 The essential factors concerned in the determination of the degree of 



* U.S. Dept. Agric, Bureau of Animal Industry, Bull. ex. pt. iii. (1914). 



