COENER — MAMMALIAN REPRODUCTIVE CYCLE 
227 
A REVIEW OF SOME RECENT WORK ON THE MAMMALIAN 
REPRODUCTIVE CYCLE 
By George W. Corner 
Workers in the physiology of reproduction, usually undertaking 
investigations to serve their practical needs as gynecologists or animal 
breeders, have found themselves under the necessity of understanding 
the reproductive cycles of some of the common domestic mammals, 
and have thus been forced to study natural history in a field but little 
cultivated by the faunal naturahsts. In the end, however, gain may 
be had in both directions, for it can hardly be doubted that important 
evolutionary trends underlie the present extraordiinary diversity of 
the reproductive phenomena of mammals. It will not be necessary 
to point out hfere extended illustrations of this diversity; we may briefly 
recall that among the ungulates alone we have such phenomena as the 
annual rutting of deer, in some species accompanied by violent oes- 
trous manifestations in both sexes; in ewes an annual season of several 
repeated heats;” in domesticated sows and cows a perennial cycle 
of about twenty-one days interval. In the laboratory rodents there 
appear to be perennial cycles of diverse lengths, with outward mani- 
festations so slight that they may evade even the practised watcher; 
while in the catarrhine apes and man we have a perennial cycle averaging 
four weeks, with what might be called a dispersed oestrus, and with 
the additional phenomenon of menstruation. Internally the same 
apparent diversity reigns; mammals differ as much in their uteri as in 
their brains, and the comparative ovarian histologist 'might almost 
duplicate some feats of the paleontologist who names a species from 
a single tooth or vertebra, for it requires but little skill to distinguish 
all the common domestic mammals by glancing at sections of their 
ovaries. These histological differences concern not only mere details 
of architecture and of size, but also the duration of growth of the corpus 
luteum, the rate and type of atresia of the Graafian follicles, and the 
amount of interstitial tissue. 
Attempts to untangle the problem have led, during recent decades, 
to the development of two general ideas about which current work is 
centering. It seems probable, first, that in all mammals the ripening 
of ova is a regularly periodic function accompanied by characteristic 
changes primarily in the ovaries and secondarily in the whole reproduc- 
tive tract; and second, that these changes are continued after ovulation 
