Morphological studies on the fetal membranes of the normal singleton foal at term.

Of 211 consecutive thoroughbred foalings, 145 satisfied a set of criteria for normal parturition and foal viability. The fetal membranes from these and from 10 pony foalings have been systematically examined morphologically and quantitatively and the findings compared to those of other authors. Five sites on the allantochorion were consistently devoid of villi. Expulsion usually occurred with the non-villous side outermost. In 24 per cent of allantochorions the non-pregant horn was of equal length or longer than the pregnant horn. These placentae tended to be shed with the villous side outermost. All membranes contained a hippomane in the allantoic cavity. Remnants of the yolk sac placenta and extra-embryonic coelom were always present. Large vestigeal yolk sacs have previously been confused with anomalous twins. In most cases the endometrial cup sites were site recognisable, usually arranged around the umbilical cord attachment. This attachment was found on the dorsal aspect of the uterine cavity either at the junction of the horns or within the pregnant hor; rarely, it was in the non-pregnant horn. A degree of twistng of the umbilical cord is normal. In 6.2 per cent of cords small urachal dilatations were noted, resulting from flight stenosis of the urachal luman at the site of a twist. Ninety-five per cent of cords measured between 36 and 83 cm.