Apr. 19,1924 Relation of Antecedent Egg Production to Sex Ratio 
215 
shown that sex is definitely established by the end of the seventh day of incuba¬ 
tion. Furthermore, F. R. Lillie (32) has discussed quite fully the embryonic 
development of the ovary and testis through the indifferent period up to the 
period of sexual differentiation, of which he says (32, p. 395): “The sex of the 
embryo can first be definitely determined about the one hundred and fifty-sixth 
hour, by the relative sizes of the gonads, by the behavior of the germinal epi¬ 
thelium, and by the presence of a larger number of primordial germ cells in the 
germinal epithelium of the female.” Minoura's statement (35, p. 34) that “this 
period, approximately the second week of incubation, is then the period of active 
sexual differentiation,” does not seem entirely in keeping with Swift's statement 
(54, p • 407) that “at the end of the seventh day of development the sex of the 
individual can be easily told * * *” It would seem that sexual differentiation 
has become definitely established by the end of the seventh day of development 
and that the accounts of subsequent development, as given by F. R. Lillie (31, 
32), Firket (15) and Swift (53, 54), involve the further development of the sexual 
organs as such. 
Minoura says that the result of his experiments shows “that sexual differen¬ 
tiation may be reversed in the chick. This result supports the explanation 
advanced by Lillie to account for the production of the freemartin in cattle.” 
But in regard to the freemartin situation in cattle, F. R. Lillie and Bascom (34) 
say that— 
In the female of cattle sex differentiation before birth is apparently due to genetic factors exclusively; 
in the male the genetic factors are intensified by the production of a hormone. 
This situation in cattle as reported by F. R. Lillie and Bascom is similar to 
that in the domestic fowl as reported by Boring and Pearl (3), Morgan (39, 40), 
Goodale (19,20), and Cole and Lippincott (4). The results of these investi¬ 
gators all go to show that the relation between the gonads and the secondary 
sexual characters is specific and not general. The results have demonstrated 
the existence of a mechanism for the control of the secondary sexual characters 
that is so closely associated with certain parts of the mechanism for the deter¬ 
mination of sex that the two go together. Hormones or chalones are secreted 
(Boring and Pearl 2, Goodale 20, and Morgan 40) which affect the development 
of the secondary sexual characters, and it would have proven very interesting 
in this connection if Minoura had grafted pieces of testis from Brown Leghorns 
on the developing embryos of Barred Plymouth Rocks and reared the chicks 
to maturity. A reversibility of sex, as claimed by Minoura, would be exhibited 
if there appeared modifications of the normal sex-linked situation in the cross 
between Brown Leghorn males and Barred Plymouth Rock females. This is 
the cross that was made in the present study, and apparently there was no 
tendency toward reversibility of sex, in spite of the variation in sex ratio in 
relation to egg production. 
The same objection may be offered with reference to Riddle's claims of sex 
reversibility in the case of pigeons. Riddle (46, p. 410) claims to have demon¬ 
strated that “germs normally female producing, have, under experiment, been 
made to develop males; and that germs which were prospectively male producing 
have been made to form female adults.” But Sturtevant (52), in connection 
with a particular cross in which Riddle claimed to have secured an excess of 
female doves due to “changing over” of the males, has suggested that the prob¬ 
able explanation rests in the selective elimination of male-producing chromosomes 
during the maturation process. Furthermore, in the present study an excess of 
females during the periods of increased egg production is certainly not due to 
“changing over” of males, there being no indication whatever of a modification 
of the characteristic plumage color of each sex in the chicks; the plumage color 
88287—241-2 
