142 DELLA DRIPS 
v lutea help in regulating the periods between oestrus. No new 
follicles become mature in the fall as long as there are luteal 
bodies in the ovary and by the time the latter have disappeared 
it is too late for the follicles to become mature before the period 
of hibernation comes on. 
A word concerning the interstitial cells may be added. These 
appear very prominent at two periods of the cycle, during the 
period of early pregnancy when the corpora lutea are young 
and apparently very active, and in the early fall when the fol- 
licles are developing and growing rapidly. Their significance is 
not evident. Several writers have claimed that the cells of the 
corpus luteum become the interstitial cells of the ovary. There 
is absolutely no basis for such a supposition as far as the spermo- 
philes are concerned. | 
EXPERIMENTAL STUDIES 
These experiments were made with a view to determining 
whether or not the results of certain experimental studies on the 
spermophile would substantiate the same work done on other 
animals. 
The spermophile stand experimental surgery well. They are 
very satisfactory to work on as they are not nearly as susceptible 
to infectious diseases and are more resistant to local infections 
than most small laboratory animals. They are easily anes- 
thetized. The surgical technic must be aseptic, and the animals 
must be placed in separate cages after the operation or they will 
chew one another’s wounds open. They recover from the effects 
of the operation quickly, as a rule, and the wounds usually heal 
by first intention. 
To all workers who have been interested in the functional 
relationship of the ovaries and the uterus and especially to those 
trying to isolate a particular function or functions for the corpora 
lutea, two problems have seemed of vital importance: First, 
what are the effects on the uterus of the removal of one or both 
of the ovaries? And, second, what are the effects on the 
ovaries of the extirpation of the uterus? ‘These two problems 
