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

field is essential.

As far as actual methods of synchronization are concerned,
there appear to be three major approaches.

First, work must continue with progestagens and other steroids,
but not on the basis of trial and error experimentation. A much
more detailed understanding of the physiological and biochemical
changes associated with oestrus, ovulation, sperm transport and sur-
vival, and fertilization is required in normal and progestagen
treated animals.

Secondly, the use of non-steroid inhibitors of pituitary
activity such as Methallibure (I.C.I. 33828) must be investigated,
on a similar basis. Such an approach has the complication, in the
ewe at least, of the necessity for progesterone priming if oestrus
is to be exhibited.

Thirdly: "there is the possible use of luteolytic agents to ter-
minate the life of an existing corpus luteum. Such agents include
the oestrogens, oxytocin, and uterine distention. The capacity of
the oestrogens to cause regression of the corpus luteum of the cow
is well established (Greenstein, Murray & Foley, 1958; Loy et al,
1960; Wiltbank, Ingalls & Rowden, 1961l; Rahlman & Cupps, 1962;
Kaltenbach, Niswender, Zimmerman & Wiltbank, 1964; Niswender,
Kaltenbach, Shumway, Wiltbank & Zimmerman, 1965; Wiltbank, 1966)
and has been used in association with progestagen to control the

cycle (Wiltbank & Kasson, 1968). Oxytocin and uterine distention
have similar effects (Armstrong & Hansel, 1959; Hansel & Wagner,
1960) . Hansel, Malven & Black (1961) have used uterine distention

in association with progesterone to synchronize cycles in cows.

More recently, Yamauchi, Nakamura & Kaneda (1965) have used the same
principle. By injecting a viscous gel through the cervix they
obtained a degree of synchronization. However there are severe
limitations to the technique, related particularly to the stage of
the cycle when treatment is commenced.

For my part, I still pin my faith on an artificial corpus
luteum, such as a subcutaneous implant or an intravaginal insert.
Such an implant or insert must release a hormone or hormones capable
of duplicating exactly the functions of the ovary and, following its
removal, events must duplicate exactly those which occur normally in
late di-oestrus, pro-oestrus and oestrus. Preferably such treat-
ment should not involve the use of supplementary gonadotrophin.

C.. Swine

The use of Methallibure (I.C.I. 33828) for synchronization of
ovulation and oestrus in the sow has been mentioned. Obviously
there are many differences between the pig and the ruminants. The
fact that oestrus occurs several days after the cessation of treat-
ment with Methallibure, and following PMS and HCG, shows the absence

1371