THE OESTROUS CYCLE IN THE MOUSE 339 
Sobotta (95) has standardized the development of the corpora 
lutea of pregnancy in the mouse, and carefully timed material 
has been available in the Washington University Anatomical 
Collection with which to check this work. Therefore, a compari- 
son of the corpora lutea of oestrus with those of pregnancy has 
been possible. Sobotta states that their persistence and ultimate 
size is not altered by conception. Surely, there is no difference 
discernible histologically during the first four days, and if their 
later size be any criterion as to the degree of their development, 
they do not reach a maximum until two new sets have been added, 
i.e., elght to twelve days after that ovulation which resulted in 
their formation. 
2. Do they inhibit ovulation? Beard (’98) has been followed 
by many investigators in advancing the idea that corpora lutea 
prevent ovulation. The fact that they are present in greatly 
hypertrophied form during pregnancy and that ovulation does 
not occur at this time seems conclusive enough proof. 
Loeb (18) first proved experimentally, by the removal of the 
corpora lutea of pregnancy in the guinea-pig after they had 
ceased to be essential to the tenure of the fetus, that a new 
ovulation could be induced earlier than would have occurred 
otherwise. He then extended the work to the corpora lutea of 
oestrus in non-pregnant animals and found the same effect of 
inhibition on ovulation. Papanicolaou (’20) has confirmed these 
experiments. The conclusion drawn was ‘‘the corpus luteum, 
itself the result of an ovulation, provides a mechanism prevent- 
ing ovulation.’”” However this may be in the guinea-pig, it is 
surely not normally applicable to the mouse during oestrus 
for (because of the shorter cycle) ovulation occurs when two or 
three sets of recent large corpora lutea are present in the ovary. 
In litter-bearing animals the corpora lutea constitute no 
inconsiderable part of the ovaries. Might not ovulation fol- 
lowing their excision be merely an expression of the compensa- 
tory hypertrophy of the remaining ovarian tissue? Corpora 
lutea of oestrus most surely do not normally prevent ovulation 
in the mouse. 
