214 
Journal of Agricultural Research voi. xxvm, No. 3 
ble that at that time sexes would be determined at hatching time, especially if 
Stretch bred the buff variety of Cochins, which were so popular in England at 
that time. Circumstances suggest to the writer that the sex of the Cochins 
was determined each year as the fowls reached maturity. The next noteworthy 
record was observed by Field (14)] in a population of 2,105 individuals the sex 
ratio proved to be 44.63. Thomsen (55) observed a sex ratio of 47.82 in a 
population of 805 Landhiihner and Orpingtons. Finally, Pearl (44) in a popula¬ 
tion of 22,791 chicks, dying in embryo and hatching, of various breeds, using as a 
basis of determination families of 10 and over, found the sex ratio to be 48.57± 
0.28. All of the sex ratios given in this paragraph were obtained on chicks 
hatched presumably during the normal hatching season. 
3. SEX DETERMINATION DURING EMBRYONIC DEVELOPMENT 
In the case of birds, where the embryo develops in the egg for some time after 
the sex is in all probability definitely established, it is obviously difficult to apply 
methods which might influence the sex after fertilization, even if it were known 
that such methods were really effective. With the exception of a few experi¬ 
ments on some of the lower animals, “there are very few cases in which there is 
even a suggestion that the sex of the fertilized egg can be modified by environ¬ 
ment, and the belief that this is possible has been entirely abandoned by many 
of the leading investigators of the subject” (Doncaster, 12, p . 147). 
Minoura (85) in an interesting study of the effects of gonad grafts in chick 
embryos, came to the conclusion that sexual differentiation is controllable, to 
some extent at least, by nongenetic factors. Using the Wyandotte breed of fowl, 
a small piece of testis or ovary was grafted on the chorio-allantoic membrane of 
developing embryos varying in age from 2 to 16 days at the time of operation. 
In a number of embryos between 5 and 13 days of age on which grafts grew and 
established vascular connections with the allantoic blood vessels, there was a 
modification of their reproductive systems. The modifications were of such 
character as to lead Minoura to conclude (85, p. 39) that “the results demon¬ 
strate that the testis and ovary produce secretions which have definite and specific 
physiological functions and which are capable of modifying the primary sexual 
characters.” That it was the secretions of the gonads which produced the 
results indicated is established by the fact that grafts of liver, spleen, thyroid, 
and thymus produced no modification of the reproductive systems in the embryos 
on which the grafts were made. Minoura concludes, therefore, that sexual dif¬ 
ferentiation may be reversed in the chick, through the action of sex hormones 
secreted by the ovary and testis. 
The development and differentiation of one sex is stimulated by the secretion of the gonad of the same 
sex and inhibited by the secretion of the gonad of the opposite sex. By means of these secretions, the dif¬ 
ferentiation of sex in the chick can be controlled to some extent. It may further be stated that the two 
sexes bear a quantitative and not a qualitative relation to each other (85, p. 88 ). 
Grafts implanted on embryos which had reached a stage of about 5 days’ 
development did not modify the embryos to any marked extent, since up to 
that stage the allantoic circulation has not been established. Embryos older 
than about 13 days are not markedly affected, since, as Minoura points out 
(85, p. 12), “it is probable that a resistance to foreign tissues has been developed 
and that such embryos are thereby enabled to destroy the grafts.” This is in 
agreement with Murphy (41), who observed that at about the time of hatching 
a defensive mechanism against foreign tissues develops in the chick. Minoura 
secured the most marked modifications of the reproductive system in embryos 
which had reached the eighth and ninth days of development, when the embryo 
“was in a relatively early stage of sex differentiation.” But Swift (58, 54) has 
