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Figure 2   There are six cellular layers that can potentially be between the fetal and maternal blood cells. Epitheliochorial (6-layer) placentas are common in pigs, cows, horses, and sheep. Endotheliochorial (4 layer) placentas are frequently found in dogs, cats, seals, and ferrets. Hemochorial placentas (where the maternal blood cells are in direct contact with the fetal chorion) are seen in humans, rats, and mice (After Renfree, 1982).
Figure 2
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