Canine placentation.
Keywords: canine, placenta, zonary, fetus, dog, bitch
An image of two canine pregnancies; one at about 39 days and the other very close to term. In the image at top, the chorion has been ruptured but, because it does not become as intimately attached to the allantois (All) as in the farm animals, the allantois remained intact here. Interestingly, even at term (see the lower image) the allantois and chorion are not firmly fused. That image shows the "halo" of the chorion, just peripheral to the allantois.
In the upper image at about 39 days, the reddish-colored yolk sac (YS) is still quite large and interestingly, is strongly echogenic on ultrasonography (more so than in horses). The amnion (Am) is intact in both of these pregnancies and in the image at top, the amnion can be seen floating, closely opposed to the fetus, adjacent to the yolk sac and allantois. The canine YS is very interesting because, unlike the situation in the mare where it is almost completely utilized by 65 days and is completely subsumed (incorporated into) by the intra-amnionic cord, the yolk sac in the canine species diminishes in size and remains in its own little cavity (an of extra-celome) up until birth. This cavity can easily go un-noticed unless one inspects the placenta closely.
