Apr. i9, 1924 It elution of Antecedent Egg Production to Sex Ratio 
203 
is the first study to be conducted on a determination of the sex ratio in the domestic 
fowl based on an entire year’s production, and it was practically impossible to use 
a large number of birds. 
The most interesting fact which Table II brings out is the high sex ratio, 
R d 1 =55.87 ±0.98, in the period of production preceding the normal hatching 
season and the low sex ratio, RcT =36.82 ±0.61, in the period of production 
subsequent to the normal hatching season. The sex ratios for each year show a 
fairly regular decrease from period to period. The only apparent exception in the 
magnitude of decrease occurs in connection with the sex ratio in period No. II, 
year 1920-21, as compared with period No. I, for the same year, and this can be 
explained by the presence of two birds giving very high male ratios, No. 191, 
Ref =91.67 and No. 197, Ref =66.67. 
The difference between the sex ratios of any two periods is significant and 
appears to be due to specific genetic or physiological factors, the probable charac¬ 
ter of which is discussed later. 
2. THE RELATION OF PRENATAL MORTALITY TO THE SEX RATIO 
Up to the present, the problem of the sex ratio in the domestic fowl has been 
considered only in terms of the relation of total numbers of sexes determinable, 
no distinction being made between the sexes of chicks that hatched and the 
sexes of the dead embryos. Notwithstanding the fact that practically every 
egg in all the families has been accounted for, the sex ratio at hatching time does 
not necessarily indicate the relative proportions of the initially produced zygotes, 
because there may have been a differential mortality of the sexes in the embryos 
which died up to the twelfth day of incubation and in the embryos which died 
between the twelfth and twenty-first days of incubation. In other words, the 
prenatal mortality may have effected a deviation from the initial zygotic ratio. 
It has been demonstrated that the mean sex ratio of the population as a who^ 
is 48.41 ±0.47 and that the mean sex ratio, based on families of 10 or more for 
the period from the commencement of laying to the end of February, is 
55.87±0.98, for the period of the normal hatching season, March and April, is 
48.82 ±0.80, and for the period from the first of May to the end of October is 
36.82±0.61. Two of these sex ratios show a marked deviation from equality 
and the problem arising out of such a situation is to determine whether a possible 
differential mortality has served as a cause in affecting the deviations. 
The first step in an analysis of this problem is to determine, if possible, the 
proportions in which zygotes of the two sexes are initially produced. It is 
obvious that the cracked eggs and the missing eggs and chicks and probably 
the infertile eggs would constitute a random sample of the population as far as 
the sex ratio is concerned. There remain for consideration, therefore, the fertile 
eggs. The sex of the zygote can not be distinguished until a certain stage of 
embryonic development has been reached, and in this study, while the sex was 
determined on some younger embryos, the author felt less possibility of error 
when using the twelfth day as the basis of earliest determination. On the other 
hand, the sex could not be determined on some dead embryos in relatively 
advanced stages of development because of degeneration of the gonads between 
the time of death and the time of dissection. But the number falling under 
this category was very small and could not materially affect either the zygotic 
ratio at the time of fertilization or the sex ratio at hatching time. 
Regarding the embrj'Os which died before the twelfth day of incubation, it is 
impossible to determine whether or not the mortality is differential. Regarding 
the embryos which died after the eleventh day, accurate records have been kept, 
which are presented in Table III, including from the commencement of laying 
