PEARL— SEX RATIO IN DOMESTIC FOWL. 433 



this ratio, and caused a deviation from the initial zygotic ratio. 

 But it is equally obvious that the post-natal mortality, whether dif- 

 ferential in respect of sex or not, can give us no direct aid in esti- 

 mating the initial zygotic ratio from the observed ratio at birth. 

 Hence the post-natal mortahty has no special interest in connection 

 with sex studies to the biologist, though it does have to the demog- 

 rapher, who is concerned, among other things, with the sex distri- 

 bution of populations throughout life. 



In poultry, the hatched chicks show a certain fairly definite 

 ratio of males to females as we have seen. Does this observed 

 ratio at birth differ from the initial zygotic sex ratio? To answer 

 this question, it is only necessary to determine whether the sex ratio 

 of the zygotes which die before hatching is, or is not, different from 

 the sex ratio of those which hatch. Theoretically this should be 

 simple. Practically it is not wholly so. The difficulty is that the 

 sex of the zygote is not distinguishable by any practical means until 

 the embryo reaches a certain more or less advanced stage of develop- 

 ment. If zygotes die before that stage of development is reached, 

 as some do, then it becomes impossible practically to determine 

 whether that particular moiety of the mortality was or was not dif- 

 ferential in respect to sex. Theoretically, of course, one should be 

 able to sex every zygote by means of a cytological examination of 

 its chromosomes. Practically, however, this is not to be seriously 

 considered. 



The result is that in the chick it is practically impossible to say 

 absolutely whether the mortality between the fertilization of the egg 

 and about the tenth day of development of the embryo is or is not 

 differential. We can, however, determine, with great precision, the 

 facts regarding the mortality from the tenth day to the end of in- 

 cubation. This has been done by the writer, during the past two 

 years. Every egg in which the embryo developed to the tenth day 

 or beyond, and died before hatching, has been opened, the embryo 

 removed and dissected, and its sex and certain other characteristics 

 recorded. This is distinctly tedious and unpleasant work, but there 

 appears to be no alternative method of getting certain sorts of in- 

 formation very essential in the analysis of many problems. 



