ABSTRACTS
44
(3) Fertility, as measured by percentage of conceptions, is generally lower in animals
in which estrus has been induced or in those forcemated after induction of ovulation,
than in animals bred during natural estrus. However, satisfactory fertility has been
reported in some cases.
Estrogens, used either alone or with gonadotropins, have given variable results.
Results of trials with gonadotropins and or estrogens during the natural breeding season
have also been quite variable. In some cases, apparent increases in fertility, as judged by
percentage of conceptions or lambing percentages, have resulted. In other cases, no
incsease or an actual lowering of fertility has resulted
The variable results are difficult to explain. The indentity of hormone preparations
can never be assured in the absence of chemical purity, and no protein gonadotropin is
now marketed in this form It does not appear probable that this accounts in any large
measure for the inconstancy of estrus response in ewes and does, owing to the relatively
uniform response of laboratory animals to these same gonadotropes.
Constitutional and environmental factors may play a considerable part in determin-
ing the response to specific hormone treatments. These factors, if they do play an
important part, are easily overcome in the induction of follicular growth and ovulation.
The extent to which they influence expression of estrus is not clear, and cannot be
fully understood until the mechanism of induction of estrus is determined in ewes and
does. A working basis for approaching this problem is furnished by Dempsey, Hertz
and Young, who advance the hypothesis that after the animal has been conditioned with,
estrogens of ovarian origin, estrus is brought about by the immediate action of
progesterone which is elaborated in the preovulatory follicles shortly before ovulation,
as a result of release of luteinizing hormone by the anterior pituitary.
The occurrence of estrus after two injections of pms. at an interval approximating
the normal cycle, indicates that the corpus luteum induced by the first injection of pms
may have a regulatory role. Also it has been observed (McKenzie and Terrill) that
most ewes ovulate at least once before the first estrous period of a natural breedpms
season. Hammond, Hammond, and Parkes state that horse ap and pms induce estring
only in animals having a regressing corpus luteum ; that in the absence of a corpus
luteum ovulation occurs without estrus, and that in the presence of an active corpius
luteum both ovulation and estrus are usually suppressed. Work by Zavadovskii,
Paduceva, and Wunder indicates that corpora lutea, induced by injection of prola no,
pms, are not in all cases similar to spontaneous cyclic ones as regards function and
capacity to regulate the sexual cycle.
From work with laboratory animals, and the results with sheep and goats, it seems
certain that time sequences in hormonal action are involved critically in the expression
of normal and induced estrus. There is obvious need for further work to determine (1)
The endocrine physiology of normal and experimentally induced estrus in sheep
and goats and (2) the specific doses and time sequences of hormones that will induce
estrus and ovulation in anestrous ewes and does.