DIOESTROUS CYCLE IN THE GUINEA-PIG 253 
changes in the uterus and finally to a degeneration of this organ. 
Yet the complete removal of the ovaries does not always pre- 
vent the menstrual periodicity from expressing itself in an 
atypicl but regular way for a considerable time afterwards 
(see Halban). 
Our observations on three females from which both ovaries 
have been completely removed, show that such an operation 
does not fully abolish the return of the destructive menstrual 
changes as is generally claimed. But on the other hand, the 
absence of the ovaries promotes and prolongs the continuation of 
these destructive changes in such a way, that instead of a periodi- 
cal menstruation, these spayed females have a long, continuous 
and atypical destruction of the uterine and vaginal mucosae, 
which leads finally to the degeneration of these organs. In some 
cases a distinct periodicity may be perceived, indicating that the 
rhythm of the menstrual activity may exist independently of 
the ovaries. The phenomenon that really is abolished and absent 
from the uterus after the removal of the ovaries is the return of 
any regenerative or reconstructive process which we believe is 
normally due to a secretion from the newly formed corpora 
lutea. 
From such a View of these phenomena one may draw the fol- 
lowing general conclusions: The oestrous changes in the uterus 
are regulated by two different factors, one direct and the other 
indirect. A secretion elaborated in the ovary apparently by the 
corpus luteum is necessary for the normal development and per- 
sistence of the uterine and vaginal mucosae. The absence of 
the secretion leads to regression and degeneration of the uterine 
tissue. Yet this control is not the entire explanation of men- 
struation. The regulation of this process and the return of defi- 
nite changes in definite periods of time may possibly be due to 
the existence of a fixed mechanism somewhere outside the ovary. 
The role of the ovary and especially of the corpus luteum is not 
to produce but to permit and to stop the menstruation. Our 
conceptions correspond completely with the ideas of Halban, 
who has recognized the protective role of the ovaries upon the 
