Vie CONG. INTERN. REPROD. ANIM. INSEM. ARTIF., PARIS, 1968, VOL. II

Secondly, the initiation of development of the corpus luteum
appears dependent on the initial release of LH. Thus Kaltenbach,
Graber, Niswender & Nalbandov (1968) found that complete hypophysec-
tomy on the day of ovulation in the ewe prevented the formation of
corpora lutea. The manner by which its subsequent function is
maintained is a matter of controversy. Whereas Kaltenbach et al
(1968) observed rapid luteal regression following hypophysectomy on
day 5, Denamur, Martinet & Short (1966) observed little effect until
after day 10. Woody, Ginther & Pope (1967) induced ovulation in
anoestrous ewes with cow anterior pituitary which is rich in LH.
They found it necessary to treat with progesterone before the time
of oestrus in order to depress the activity of the ensuing corpus
luteum. This is in contrast with the situation in the normal
cyclic ewe and raises the question of the nature of the gonadotro-
phin and its LH content in relation to the suppressive effect of
administered progestagen.

Thirdly, there may well be two hypothalamic centres involved in
the release of LH, one involved in the steady release of tonic
levels, and the other with the precipitate or "cyclic! release
involved in ovulation and the initiation of luteal activity. Work
which led to this concept has been reviewed by Flerko (1963). There
is no evidence of a progesterone feed back mechanism affecting
luteotrophic activity, once the corpus luteum has been formed. Thus
McDonald & Clegg (1967) found no effect of progesterone on circula=
tory levels of LH, presumably that controlled by the "tonic!" centre
(tonic release). However cessation of progesterone was followed by
a rise in LH some 5 days later, presumably through release of the
'cyclie" centre;

Finally, in the cow at least, LH, presumably that released
under the control of the "tonic" centre, appears to be involved in
the maintenance of the functional activity of the corpus luteum
(Mason & Savard, 1964; Simmons & Hansel, 1964; Donaldson & Hansel,
1965a, b; Hansel, 1966, 1967; Hansel & Seifert, 1967).

There is clear evidence that progesterone prevents the release
of "cyclic" LH responsible for ovulation, luteinization and luteal
function. The questions which require definitive answers are:

(a) Does progesterone also inhibit the release of FSH?

(b) Does FSH stimulate the production of oestrogen?

The ovaries of sheep under the influence of exogenous proges-
tagen show no evidence of follicle stimulation, and the data
presented graphically in Figure 3 clearly suggest the involvement
of FSH in the production of oestrogen. Hence it seems reasonable
to conclude that, for the sheep at least, progesterone does inhibit
the release of FSH and that FSH is responsible for the production
of oestrogen. Also, let us assume that oestrogen is involved in

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