The pig presents the simplest type of diffused placenta. The villi of the surface of the chorion cover a broad zone, leaving only the two poles free; their arrangement differs therefore from that in a zonary placenta in the greater breadth of the zone covered by them. The villi have the form of simple papillæ, arranged on a series of ridges, which are highly vascular as compared with the intervening valleys. If an injected chorion is examined (fig. 154), certain clear non-vascular spots are to be seen (_b_), from which the ridges of villi radiate. The surface of the uterus adapts itself exactly to the elevations of the chorion; and the furrows which receive the chorionic ridges are highly vascular (fig. 155). On the other hand, there are non-vascular circular depressions corresponding to the non-vascular areas on the chorion; and in these areas, and in these alone, the glands of the uterus open (fig. 155 _g_) (Turner). The maternal and foetal parts of the placenta in the pig separate with very great ease. [FIG. 156. VERTICAL SECTION THROUGH THE INJECTED PLACENTA OF A MARE. (From Turner.) _ch._ chorion with its villi partly _in situ_ and partly drawn out of the crypts (_cr_); _E._ loose epithelial cells which formed the lining of the crypt; _g._ uterine glands; _v._ blood-vessels.] In the mare (Turner), the foetal villi are arranged in a less definite zonary band than in the pig, though still absent for a very small area at both poles of the chorion, and also opposite the os uteri. The filiform villi, though to the naked eye uniformly scattered, are, when magnified, found to be clustered together in minute cotyledons, which fit into corresponding uterine crypts (fig. 156). Surrounding the uterine crypts are reticulate ridges on which are placed the openings of the uterine glands. The remaining Ungulata with diffused placentæ do not differ in any important particulars from those already described.