REPRODUCTION IN DOMESTIC FOWL 109 



suggests that we may have here a result due to heterosis, of the 

 same sort essentially as those which have been observed partic- 

 ularly by East^ in plants. 



The coefficients of variation for these ovarian counts fall, in 

 the longer and hence more reUable series, in the neighborhood of 

 a value of 30 per cent. This is about the value found for the 

 relative variability of a number of physiological characters, 

 particularly fecundity of the domestic fowl as measured by 

 annual egg production.^ 



RELATION OF OOCYTE NUMBER TO AGE 



It is a weU-known fact th at in the domestic fowl egg production 

 rapidly diminishes after the first year or in some breeds such as 

 the Leghorns the first two years of Ufe. Fowls may live for a 

 considerable number of years, but generally after the early years 

 they lay no eggs at all, or at most one small clutch in the spring. 



In the case of Barred Plymouth Rocks, of the Maine Station 

 strain, egg production usually practically ceases after the bird is 

 two years old. It is commonly assumed that this cessation or 

 great reduction in fecundity with age is due to atrophic processes 

 in the ovary. It is, on this account, of interest to see in what 

 manner the counts of visible oocytes change with age. 



Unfortunately, the material in which the point can be tested 

 is meager. The Barred Plymouth Rocks constitute the only 

 group sufficiently large, and there is a highly uneven distribution 

 by age. Of the thirteen birds, nine were under one year of age 

 when killed, one was between one and two, two between two and 

 three, and one between three and four. It will be worth while, 

 however, to see what this admittedly meager material shows. 

 The data are exhibited in table 3 and graphically in figure 2. 



In so far as the data may be trusted as fairly representative 

 of what generally occurs, they indicate that the number of visible 

 oocytes increases with age, within the age Hmits of the observa- 



^Cf. East, E. M., and Jones, D. F. Inbreeding and outbreeding. 1919. 



^Cf. Pearl, R., and Surface, F. M. A biometrical study in egg production in 

 the domestic fowl. I. Variation in annual egg production. U. S. Dept. of Agric, 

 Bur. of An. Ind. Bull. 110, Part I. 



