216 
Journal of Agricultural Research voi. xxviii. No. 3 
is always inherited in a definite sex-linked Mendelian manner. Also, only in 
very few cases did there appear to be an abnormal condition of the gonads at the 
time of dissection. 
Two cases concerning assumed reversibility of sex have been reported recently, 
Crew (5) in the case of a Buff Orpington chicken and Riddle ( If.8 ) in the case of a 
pigeon. The case reported by Crew deserves special mention, inasmuch as he 
placed undoubted reliance upon the statements of the original owner of the 
chicken concerning its apparent sex up to three and one-half years of age. The 
chicken is stated by Crew to have been “a purebred Buff Orpington, a good 
layer and a mother of chickens,” although Crew makes no mention of the bird 
laying eggs after he secured it. It is very unfortunate, therefore, that Crew was 
not able to secure undisputed evidence concerning the original sex of the chicken. 
Suppose, however, that the chicken was originally a female with a sex chromo¬ 
some constitution of XY. Suppose also that sex reversal actually occurred, 
spermatozoa of XY constitution being produced, as Crew suggests. Such a bird 
when mated to a normal female should give a sex ratio of 33.33, since the YY 
chromosome complex constitution would not develop. Now, if the normal female 
had possessed the sex-linked barring factor, then half of the female progeny 
should have been black and half barred instead of all being black, as in the case 
of the present study. Crew was able to raise only two chickens from the mating, 
one male and one female, numbers too small to indicate the sex chromosome 
constitution of the chicken for which it is claimed the sex became reversed. 
The results of the present study involving several matings and several hun¬ 
dred chicks from mothers possessing the sex-linked barring factor have provided 
no evidence of change in sex during embryonic development. Cases of reputed 
sex reversibility where all of the evidence may be unquestioned concerning the 
sex of the individual from hatching time to the time sex is supposed to have 
been reversed are awaited with considerable interest. 
Finally, it may be said that it is possible that the sex of an embryo may be 
modified after fertilization if the physiological condition on which sex depends 
can be changed. Based upon the results of the present study the statement 
may be made that until more convincing evidence is available it seems necessary 
to conclude that sex determination is affected only at or prior to fertilization. 
4. SEX DETERMINATION AT FERTILIZATION 
The process of fertilization is uniform in all its essentials throughout the 
animal kingdom. It consists in the union of two nuclei derived usually from 
two parents, with the mingling and division of their substance “in such a way 
that the chromosomes from each parent are equally represented in both the 
daughter nuclei produced by the first segmentation division” (Doncaster, 13 t 
p. 117). The process of fertilization serves as about the only definite indication 
of the nature and function of sex. Since there are only two distinct sexes and 
since in any species the average ratio in which they are produced is approxi¬ 
mately constant, it is obvious that the mechanism for producing the equal divi¬ 
sion of chromosomes is of fundamental significance. Jennings {27, p, 193) has 
stated the problem simply and clearly: “The really fundamental thing that 
mating does is to produce new combinations of hereditary characters.” 
The statement made above, concerning the equal representation of the chro¬ 
mosomes in the daughter nuclei, is of a general character, and does not apply to 
those cases where one sex is homozygous and the other heterozygous for the sex 
character. In the case of the domestic fowl, it has been shown previously from 
cytological observations that the female is heterozygous and the male homozy¬ 
gous for sex. This fact is amply supported by the breeding results secured by 
