a. Hormone assays

Modern assay techniques are improving so rapidly that within
the next decade we should have a clear picture of endocrine levels

and interactions during the oestrous cycle, The present state of
knowledge, as it relates to the sheep, has already been discussed.
Comparable data for the cow are not yet available. Earlier work on

the methods used and results obtained for progesterone has been
reviewed by Gomes & Erb (1965), and little has emerged since other
than the attempts by Gupta & Pope (1966) and Pope, Jones & Waynforth
(1965) to measure peripheral progesterone and oestrogen.

b. Indirect estimates

i. Morphology, histology and histochemistry.

Ovaries: In the sheep the time of ovulation
relative to the onset of oestrus and the pattern of development of
the follicle, its rupture and the formation of the corpus luteum and
the general appearance of the ovaries appears perfectly normal
following progestagen treatment (Robinson & Smith, 1967).

In the cow, by contrast, the pattern may differ, Thus Trim-
berger & Hansel (1955) and Smallwood & Sorenson (1967) observed an
increased incidence of cystic follicles following MAP. Ludwick,
Wilson & Ames (1966) suggest that CAP leads to an altered pattern of
development of follicles, Zimbelman & Smith (1966) found that
treatment with MGA resulted in enlarged ovaries due to excessive
follicular fluid and attributed this to inadequate suppression of
pituitary FSH activity.

The reproductive tract: The observations of Small-

wood & Sorenson (1967) and Quinlivan (1967) have been mentioned. 1In
addition to the histology of the tract, Quinlivan carried out some
preliminary studies on alkaline phosphatase and glycogen using the
histochemical techniques of GOmoéri (1952) and Lillie (1952).
Differences between untreated and treated animals were small
compared with the following two major histological changes. First,
clear degeneration and subsequent desquamation of the glandular

epithelium commenced on the 13th day of the normal cycle. These
changes reached a peak on day 15, were still evident on day 16 but
were resolved at oestrus. In ewes treated with Cronolone impreg-

nated sponges, comparable degenerative changes commenced 2 days
before withdrawal of the sponges and reached a peak by 24 hours
after withdrawal, regardless of the stage of the cycle when sponges
were inserted. Resolution of these changes appeared less rapid
than normal so that on the day of oestrus a large proportion of
glands were occupied by cellular debris,

Secondly, leucocytes were virtually absent at oestrus in

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