INHERITANCE OF FECUNDITY IN DOMESTIC EOWI,. 375 



As has been poiiited out at the beginning of this section of 

 the paper, there undonbtedly exist differences in what might be 

 called physiological compatibility between fowls of different 

 genotypic constitutions. Some concrete data regarding this 

 have been published in a previous paper (39). More will be 

 presented later. While some gametic combinations (at least at 

 their first synthesis) lead to physiologically weak and abnormal 

 individuals, others produce individuals which in vigor, rate of 

 growth, etc., surpass the normal. This phenomenon is perhaps 

 more clearly and strikingly shown in pre-natal mortality (em- 

 bryos dead in shell) than by any other character in fowls which 

 can readily be measured. I hope shortly to publish a paper on 

 this subject, and will only anticipate that paper here to the 

 extent of saying that all the experience in this laboratory with 

 cross-bred poultry agrees in showing that while there may be 

 differences in the ease or success with which fertilization of 

 the egg occurs in breed crosses, these are relatively unimportant 

 as compared with the differential embryonic mortality. The 

 proportion of embryos which start to develop, but lack the 

 power to complete development is uniformly much greater for 

 some hereditary combinations than for others, regardless of the 

 particular individuals used as parents, and under uniform condi- 

 tions of incubation and of housing and feeding the parent stock. 



It is in this direction that I am inclined to look for the explan- 

 ation of the discordant results of S 578's matings with barred 

 Fi females. The records give one the impression that the poten- 

 cy or absolute fecundity value of the several gametic factors 

 had, because of the super-normal physiological condition, been 

 bodily raised considerably above the normal for the strains used 

 in these experiments. One cannot escape the feeling that all 

 these birds were making higher records than individuals of the 

 same gametic constitution but of more ordinary physiological 

 character in general would have done. The scale of fecundity 

 values has apparently shifted in an upward direction; in other 

 words something similar to what occurs when two inbred strains 

 of maize are crossed happened here. 



Along this line is the only explanation for the outcome of 

 these four matings that I am able to suggest. It is quite possi- 

 ble that it may have no bearing, and that the results are due to 

 some peculiarity of gamete formation which can be suggested 



