PEARL— SEX RATIO IN DOMESTIC FOWL. 435 



days of incubation. But there remains a certain mortality during 

 the first ten days. We are in position to say, on the basis of evi- 

 dence already given, that in the Maine Station flock male and female 

 zygotes are present in the proportion indicated by 7^^=48.5 at the 

 time when the zygotes are 10 days old. Were they initially present 

 in equal numbers and did enough more males than females die dur- 

 ing the period to the tenth day of incubation to produce the /?c? = 

 48.5 status? Here we would call attention only to two points. 

 The first is that in the flocks which have furnished the statistics here 

 dealt with, the rate of prenatal mortality before the tenth day of in- 

 cubation has always been low — so low that if differential mortality 

 within this period is to be adduced as the explanation of the ob- 

 served sex ratio, it would be necessary to assume that practically 

 every embryo which died within these first ten days was male. A 

 theory can only be regarded as highly improbable which demands 

 that during any period of life all naturally occurring deaths are 

 of individuals of the same sex, wdien it is known to be the fact 

 that in all other periods of life the individuals of the two sexes die 

 in numbers roughly proportional to the numbers living of each sex. 



In the second place, it is in the highest degree improbable that 

 there is an abrupt change in the mode of incidence of the mortality 

 with respect to sex at exactly the tenth day of incubation. Yet 

 such an abrupt change would be demanded by any theory which 

 makes differential mortality the explanation of the observed sex 

 ratio in the fowl. From the time when the embryo has developed 

 sufficiently to make it possible certainly to distinguish the sexes in 

 poultry by macroscopic examination of the gonods, we know that 

 the mortality is either not dift'erential at all with respect to sex (pre- 

 natal mortality), or is at most only slightly so (possibly so in post- 

 natal mortality though the point has not been fully investigated yet). 

 In the absence of any evidence favorable to such a view, it could 

 only be regarded as a highly improbable speculation to say that in 

 the ver}^ earliest stages of embryonic development all deaths are 

 males. 



We are justified, I think, in concluding that in the flocks of 

 poultry here dealt with, and probably in the fowl generally, that 



