CORNER — MAMMALIAN REPRODUCTIVE CYCLE 
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butions sharpening our knowledge of the relation between ovulation and 
oestrus in animals showing marked outward signs of heat, as for instance 
those of Longley (1912) on the cat, Lewis (1911) and Corner and Ams- 
baugh (1917) on the sow, and Kiipfer (1920) on the cow. In all these 
animals oestrus is promptly followed by the development of corpora 
lutea in the ruptured Graafian follicles. 
The second general idea outlined above has slowly grown out of the 
suggestion of Prenant and Born that the corpus luteum is a gland of 
internal secretion, serving to produce changes in the uterus and ovary. 
Later workers, including Fraenkel, L. Loeb, Ancel and Bouin, Hill and 
O’Donoghue, and others, have developed the hypothesis by assuming 
that the changes effected by the corpus luteum are aimed at facilitating 
the implantation of the embryos. The best experimental attack on 
the problem was made in 1907 by Leo Loeb, who found that (in the 
guinea-pig) when newly-formed corpora lutea are present in the ovaries, 
and then only, the uterine mucosa is in a specially responsive state, 
so that the presence of the ova, or even of an artificial foreign body, 
leads to the production of a decidual or placenta-like change of the 
uterus. In its simplest form this beautiful experiment, now several 
times confirmed by others, requires only that one select an animal 
about one week after an unfertilized ovulation, and traumatize the 
uterine mucosa with a needle. Four or five days later the stimulated 
areas are marked by the presence of swellings histologically resembling 
decidual tissue. Preliminary removal of the ovaries (or, according 
to Loeb, of the corpora lutea alone) prevents the formation of the 
deciduomata. 
The present writer has attempted, by way of testing the foregoing 
considerations, to study the whole ovarian and uterine cycle in one 
species, and to make the supposed utility of the uterine changes 
for implantation of the embryos something more than a matter of 
hypothesis, by actually correlating the state of the uterus at every 
stage with the progress of the ova and embryos. For this purpose 
the domestic sow was chosen, because it appears to be the only 
mammal whose earliest embryology and mode of implantation are 
as yet sufficiently known and accessible to re-study which at the 
same time exhibits uncomplicated and outspoken oestrous phenomena. 
It has been found, in brief, that the external manifestations of 
oestrus, which recur at intervals of about twenty-one days, are asso- 
ciated with a regular ovarian cycle. A day or two before thp onset 
of oestrus a group of follicles is prepared for ovulation; they rupture 
