Now in individuals which are high layers, and have this characteristic in hereditary form, there must be involved some sort of physiological factor in addition to the normal ovulation factor already discussed. An analysis of extensive statistics has shown that high fecundity 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 physiological 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 of 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 (i. 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,
