THE INHERITANCE OF FECUNDITY 367 



Now in individuals which are high layers, and have this character- 

 istic in hereditary form, there must be involved some sort of physio- 

 logical factor in addition to the normal ovulation factor already dis- 

 cussed. An analysis of extensive statistics has shown that high fecun- 

 dity represents essentially an addition of two definite seasonal, laying 

 cycles to the basis normal reproduction cycles. These added periods of 

 productivity are what may be called the winter cycle and the summer 

 cycle. The winter cycle is the more important of these. It is the best 

 measure of relative fecundity which we have and has been used as the 

 chief unit of fecundity in these studies. It constitutes a distinct and 

 definite entity in fecundity curves. The existence of these added 

 fecundity cycles in high laying birds must depend upon some additional 

 plrysiological factor of mechanism besides that which suffices for the 

 normal reproductive egg production. Given the basic anatomical and 

 physiological factors, the bird only lays a large number of eggs if an 

 additional factor is present. 



We may next consider in greater detail these factors influencing 

 fecundity, taking first 



The Anatomical Basis op Fecundity 



Since, as already pointed out, egg production obviously depends in 

 part upon the presence of ova in a normal ovary, a question which 

 demands consideration is the following: 



To what extent are observed variations in fecundity (?'. e., in the 

 number of eggs laid) to be referred to anatomical differences? In 

 other words, does the ovary of a high-producing hen with, for example, 

 a winter record of from 75 to 115 eggs contain a larger number of 

 oocytes than does the ovary of a hen which is a poor producer, laying no 

 eggs in the winter period and perhaps but 10 or 15 eggs in the year? 



To get light upon this question the observations to be described have 

 been made. The object was to arrive at as accurate a relative judgment 

 as possible regarding the number of oocytes in the ovaries of different 

 individual birds. It is, of course, impossible practically to determine 

 accurately the total absolute number of oocytes in the ovary. What 

 can be done, however, is to count the number of oocytes which are 

 visible to the unaided eye. While such results do not tell us, nor enable 

 us to estimate with great accuracy, the total number of oocytes in the 

 ovary, they do, nevertheless, throw interesting and useful light on the 

 questions raised above. Some counts of this kind are shown in Table I. 



From this table it is in the first place clear that the number of 

 oocytes in the ovary of a hen is very large; much larger, I think, than 

 has generally been supposed. While, to be sure, there are for the most 

 part only vague statements respecting this point in the literature, 



