No. 614] 



EGG PRODUCTION 



71 



their effect is exerted only indirectly. They undoubtedly 

 play an important part in egg production, but like many 

 external factors they are without influence unless they 

 fail in some way. Such factors are the capacity to digest 

 and assimilate food, to excrete waste matters properly, 

 etc. It is not my purpose at this time to discuss such 

 factors. Those internal factors with which we are mostly 

 concerned are those whose relations to egg production are 

 much more obvious. They are rate of growth of the 

 chick, cessation of growth, the attainment of both bodily 

 and sexual maturity, moults, the size of the bird, the 

 stamina of the bird, the presence or absence of cycles, 

 litters or clutches of production, the rhythm of produc- 

 tion, the rate of production for definite time intervals, 

 age at first egg, and broodiness. Some of them are 

 clearly separable from egg production. Others are so 

 closely interwoven that it is impossible to say that they 

 are not phases of egg production. Whether or not this 

 is so, is of no immediate importance from the standpoint 

 of inheritance, since the result will probably be the same 

 whether they are treated as genetic factors that are sep- 

 arable from egg production or treated as groups into 

 which egg production itself may be divided. These fac- 

 tors may be regarded as phases of egg production if one 

 desires, but on the whole it has seemed profitable to re- 

 gard them as factors influencing egg production. 



Rate of Groiuth, Bodily Maturity, Cessation of Growth, 

 Sexual Maturity 1 

 These interrelated factors are closely interwoven in 

 their effect on egg production. Under normal conditions 

 it is clear that sexual maturity is indicated by the begin- 

 ning of egg laying, and may be measured by a bird's age 

 at her first egg, i. e., the length of time elapsing between 

 the date hatched and date of first egg. Sexual maturity, 

 however, demands certain antecedent conditions before it 

 can become manifest. Among other conditions is a cer- 



