operate in the progestagen treated sheep.

In perspective, ‘it seems that cervical penetration is not in-
volved, at least in the sheep. Motility of the tract is an unknown
quantity, but the limited work with oxytocin on the cow and sheep
(Hansel, 1967; Lightfoot, 1968) suggests that this is unlikely to

be a major factor. Hence the most likely causes of failure are
faults in the tract leading to excessive voidance or destruction of
spermatozoa, These faults almost certainly have an endocrinolo -

gical basis, complicated perhaps, in the case of intravaginal
treatment, by physical or microbiological factors.

d. Reasons for faults in the tract

i. Physical faults. These are most likely to be a
factor in early lactation due to incomplete involution, retention of
debris, and infective conditions resulting from parturition. Vag-
inal damage or infection following the use of intravaginal sponges
could also be a factor in some cases.

ii. Physiological faults. There is a great deal of

evidence concerning the critical nature of the oestrogen-progestagen
interaction on the physiological state of the reproductive tract and
on fertilization in laboratory mammals. Almost certainly we are
dealing with similar phenomena in our domesticated animals. An
imbalance in this delicate relationship is almost certainly the most
important factor in the sub-fertility associated with progestagen
treatment and is discussed in some detail below.

e. Reasons for embryonic mortality

The case for the involvement of early death of the embryo
appears to be confined to early lactation, and this also will be
discussed later.

2., Endocrine status at oestrus

The critical period is the 48 hours centred about the time of
onset of oestrus, and Figure 3 illustrates the sequence of endoc-
rinological events about this period in the normal cyclic sheep.
Temporal and quantitative factors are vitally important.

Following are the steps:

(a) Some 48 hours before the expected time of oestrus the output of
progesterone by the corpus luteum begins to fall rapidly. By
24 hours it is negligible.

(b) Some 16 hours after progesterone reaches negligible levels -
that is about 8 hours before oestrus - FSH is released. This
continues for about 14 hours, by which time about 50 percent of

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