ZOOLOGY: F. R. LILLIE 
465 
whether such differentiation is more or less controllable, or even com- 
pletely reversible? 
Up to a certain stage in the development of mammals there are no 
morphological evidences of the determined sex. Prior to this stage 
sex-characters are identical in both kinds of zygotes; the gonad first 
enters on a phase of male differentiation in both sexes, which subse- 
quently changes to the female direction in the female zygotes only; 
both male and female sex-ducts arise in each kind of zygote, and by 
subsequent corelative progressive and retrogressive differentiation the 
conditions of the appropriate sex are produced; external parts also 
appear similarly at first in both sexes. The possibilities for complete 
reversal of the indicated sex-differentiation would therefore seem to lie 
within this so-called sexually indifferent stage, and to diminish pro- 
gressively as differentiation proceeds. 
There are many indi ations that each zygote, whether determined as 
male or female, has both tendencies, both sets of sex-factors; in other 
words that the reactions for male or for female differentiation are both 
possible for each zygote; but that they are to a considerable extent 
mutually exclusive in bisexual animals. The initial sex-determination is, 
therefore, a condition in which there is a quantitative superiority of one 
or the other tendency or set of factors. The advance of development 
progressively limits the possible operations of the inferior set of factors, 
so that, by both positive and negative limitations, reversal of the initial 
sex-index becomes increasingly more difficult. 
It has long been known that the degree of development of the sex- 
characters that arise after birth is dependent in mammals upon internal 
secretions of the sex-glands (sex-hormones) circulating in the blood. 
This is seen in the well-known effects of castration ; and partial reversal 
of sex-differentiation has been secured by implantation of the sex-gland 
of the opposite sex following castration.^ But the more fundamental 
sex-characters, like other fundamental characters, are differentiated 
in mammals before birth. Such are the type of sex-gland, whether 
ovary or testis, the type of sex-ducts, whether vasa deferentia or female 
reproductive tract, and the type of the external organs of reproduction. 
The problem of the extent to which sex-differentiation may be re- 
versible carries us back therefore to the sexually indifferent stage, and 
the question arises whether the sex-characters that develop before birth 
are, Hke those arising after, dependent for the degree of their develop- 
ment upon sex-hormones, and whether, like the latter also, they are more 
or less reversible by action of the sex-hormones of the opposite sex? 
