436 Experimental Zoology 
female develops; if the latter, a male. We lack the evidence 
to establish such a view at present, and our more immediate 
problem is first to discover what factors determine the sex of 
the individual. 
So far we have considered only the effects of removal of the 
essential organs of reproduction, the ovaries and testes, on the 
general characters. It has been shown for the males that 
the removal of these organs in youth also affects the acces- 
sory parts of the reproductive organs, — the glands and ducts. If 
the testes are removed from the adult mammal, there is a decrease 
in the size of the prostate gland that may be due to functional 
atrophy, or may be due to more direct influences. If the testes 
are removed from a young animal, the prostate does not develop 
further, and remains quite small. One-sided castration pro- 
duces no effect, either on the same or on the opposite side. The 
development of Cowper’s gland seems to be correlated with the 
development of the prostate, and after castration remains un- 
developed. Other parts of the accessory system also fail to 
develop completely after castration. 
It is generally assumed that the influence arising from the 
testes must be in the nature of an internal secretion, which, set- 
ting free certain materials in the blood, affects the development 
of remote parts of the body. In support of this view is the fact 
that the influence is not unilateral, but general. There is a 
further result that shows how important a réle the internal secre- 
tions of the reproductive organs may play in certain changes in 
other parts of the body. It has been found by Frankel and 
Cohen that the presence of the cup-shaped corpora lutea, left 
on the surface of the ovary after the egg has escaped, is necessary 
for the fixation of the egg to the wall of the uterus. The egg or 
young embryo fixes itself six days after it has been set free from 
the ovary. Ifthe ovary is removed at this time, the fixation does 
not occur. Even if the ovary is left, but the corpora lutea are 
destroyed by a galvanic needle, the fixation fails to take place. 
It is probable that the corpus luteum is a gland, as its structure 
suggests, and the substance produced influences the wall of 
