of dependence upon progesterone priming for the demonstration of
oestrus - a conclusion reached by Signoret (personal communication)
from work on the spayed sow. It seems that the sow in pro-oestrus
and oestrus is entirely oestrogen-dependent and this is reflected in
characteristics such as the enzymic activity of the endometrium.
Although recent field tests show that a high degree of synchroniza-
tion and acceptable fertility are obtainable using Methallibure in
association with PMS and HCG, the method is not ideal. Methallibure
is fairly toxic and its use requires that of exogenous gonadotrophin.
There is still a place for basic work on the oestrous cycle of the
sow and for its control by methods which do not involve the injec-
tion of gonadotrophins.

D. Mare

Very little has been done on the mare since the early demonstra-
tion that ovulation could be induced and the duration of oestrus
shortened by the injection of HCG (Mirskaya & Petropavlovskii, 1937;
Day, 1939; McKenzie, 1940; Davison, 1947).

Recently, Loy (1966) has been studying the effects of proges-
tagens in mares, and has found that MAP is ineffective in blocking
oestrus and ovulation at very high doses. Progesterone, on the
other hand, is effective at dose levels of 100 mg per day. These
observations are particularly interesting in the light of those of
Zimbelman (1966) on the failure of MGA to inhibit the release of FSH
in the cow, bearing in mind the very high FSH content of the horse
pituitary. They suggest, also, qualitative differences between
progestagens in common use.,

III. THE NON-BREEDING SEASON

This is a problem primarily of sheep and goats. Basically,
there are three methods of inducing ovarian activity in anoestrus.
In fundamental order they are

(a) Alteration of the photoperiodic environment (Yeates,
1949) .

(b) Injection of gonadotrophin (Cole & Miller, 1933).
(c) Introduction of rams (Underwood, Shier & Davenport, 1944).

Of these, only the first and second are applicable to the mid-
anoestrous period, and only the first will induce a series of
cycles, The use of progesterone prior to gonadotrophin will result
in oestrus with the induced ovulation (Dauzier & Wintenberger, 1952;
BUBE 1952 SBeRobinson, . 1952)., If such treatment is near the end of
the anoestrous period, a series of cycles may be initiated with a
consequent advancement of the breeding season (Gordon & Keane, 1967;
Robinson & Smith, 1967).

1372