STUDIES ON THE PLACENTA OF THE SHEEP (OVIS ARIES L.)

Abstract

A positive correlation is reported between birth weight and the weight of intact cotyledons and of foetal cotyledons. At equal cotyledon weight, single foetuses are heavier than twins, male foetuses are heavier than females and those of older ewes tend to be heavier than those of younger ones. There is great variability in the total weight of cotyledons per ewe, even within groups of similar ewes. The relationship between foetal weight and cotyledon weight appears to be curvilinear near parturition, increases in foetal weight being associated with increasingly large increments in cotyledon weight. Foetal weights of about 1.5 kg are achieved at very low cotyledon weight. There is only slight correlation between foetal weight and number of cotyledons, because of a compensatory increase in the weight of individual cotyledons when they are few in number. This involves both foetal and maternal tissue. The number of cotyledons per foetus varies with breed and strain of sheep, litter size, age (or parity) of ewe, sex of lamb and with environmental conditions and the number of caruncles in the uterus. There is a marked tendency for a constant proportion of the caruncles (70 to 80%) to develop into cotyledons. Fine and medium wooled Australian Merinos have fewer cotyledons than any other breed for which data are available. In the sheep, as in man, a number of very small foetuses, with very small placentae, reach full term.