156 RAYMOND PEARL 



mals it will obviously be very difficult, if not impossible, to get reliable 

 quantitative data regarding pure fecundity. On the other hand we 

 would suggest that the term 'fertility' be used to designate the total 

 actual reproductive capacity of pairs of organisms, male and female, as 

 expressed by their ability when mated together to produce (i.e., bring 

 to birth) individual offspring. Fertilit}^, according to this view, depends 

 upon and includes fecundity, but also a great number of other factors in 

 addition. Clearly it is fertility rather than fecundity which is measured 

 in statistics of birth of mammals. 



Taking fecundity as above defined it is obviously a character 

 depending upon the interaction of several factors. In the first 

 place the number of ova separated from the body by a hen must 

 depend, in part at least, upon an anatomical basis, namely, the 

 number of ova present in the ovary and available for discharge. 

 Further there must be involved a series of physiological factors. 

 The mere presence of an anatomically normal reproductive sys- 

 tem, including a normal ovary with a full complement of ova, and 

 a normal oviduct, is not enough to insure that a hen shall lay 

 eggs, that is, exhibit actual as well as potential fecundity. While 

 comparatively very rare, cases do occur in which a bird possesses 

 a perfect ovary and perfect oviduct and is in all other respects 

 entirely normal and healthy, yet never lays even a single egg in 

 her life time. Such cases as these prove (a) that what we may call 

 the anatomical factor is not alone sufficient to insure that poten- 

 tial fecundity shall become actual, and (6) that the anatomical 

 and physiological factors are distinct, in the sense that the normal 

 existence of one in an individual does not necessarily imply the 

 co-existence of the other in the same individual. 



A case of this kind is found in hen no. 8051 hatched March 29, 

 1909, and killed for autopsy record August 24, 1911. This bird 

 had the secondary sexual characters of the female perfectly 

 developed, and was entirely normal in other respects (body 

 weight, 2366 grams). This bird never laid an egg during its life. 

 The ovary w^as normal (fig. 1) and was of about the size proper to 

 a fully developed pullet just reaching the point of beginning to 

 deposit yolk rapidly in certain oocytes in preparation for laying. 

 While counts were not made this ovary appeared to carry a nor- 

 mal number of oocytes. In general it was anatomically normal. 



