230 
JOURNAL OF MAMMALOGY 
during the oestrous period, and are succeeded by corpora lutea, which 
reach their full development in one week, and remain in active state 
until the fifteenth day after ovulation, when they suddenly degenerate 
to make way for a new group of follicles. If the ova are not fertilized 
they pass into the uterus and degenerate there about the seventh or 
eighth day; but if fertihzed they become attached to the uteritie mucosa 
between the tenth and fifteenth days, at the very time when the corpora 
lutea are at their height. (In this case, of course, the corpora lutea do 
not degenerate, but persist throughout pregnancy.) There is also a 
parallel series of changes in the uterus. At oestrus the uterine mucosa is 
in a state like that described in the rodents by Stockard and Papain- 
colaou and by Long and Evans; but during the following week it under- 
goes marked growth changes, and during the second week (the time 
of implantation) still further histological modifications take place 
which seem clearly adapted to aid first in transporting and then in 
attaching the embryos. These alterations go on, however, whether 
or not the ova are fertilized; but if no embryos are formed, then when 
the corpora lutea degenerate the uterine mucosa also reverts to its 
original condition, and by the simultaneous occurrence of epithelial 
degeneration and proliferation is brought back to the oestrous stage. 
Long and Evans (1921) find that in the albino rat there are charac- 
teristic changes of the vaginal mucosa during the first weeks of preg- 
nancy. Under the conception which we have been outlining, similar 
changes shbuld occur after an unfertilized ovulation, but it appears 
that their full development is aborted by the briefness of the cycle in 
this species. However, as Long and Evans have discovered, if the 
return of oestrus be postponed by the insertion of a glass rod into the 
cervix uteri, as described above, then the vagina shows changes in 
every way similar to those of pregnancy. 
It will be apparent from the foregoing review that this branch of 
investigation has now reached a stage of exploration and of compari- 
son, and that there is a pressing necessity for careful study of the 
reproductive habits of all accessible mammals. From the medical stand- ^ 
point an especial interest attaches to the mechanism of reproduction 
in the primates, and it is here that the greatest service can be rendered 
by those who deal with wild animals at large and in captivity, by gather- 
ing together enough information to give a basis for experimental work. 
We have seen that this work must be founded upon specific knowledge 
of life-processes; at present we have little more than hearsay and trav- 
ellers’ tales about the reproductive cycle of tl^ monkeys and apes. 
