The groups not belonging to the Ungulata which are characterized by the possession of a diffused placenta are the Sirenia, the Cetacea, Manis, and the Lemuridæ.
Sirenia. Of the Sirenia, the placentation of the Dugong is known from some observations of Harting (No. [201]).
It is provided with a diffuse and non-deciduate placenta; with the villi generally scattered except at the poles. The umbilical vesicle vanishes early.
Cetacea. In the Cetacea, if we may generalize from Turner’s observations on Orca Gladiator and the Narwhal, and those of Anderson (No. [191]) on Platanista and Orcella, the blastodermic vesicle is very much elongated, and prolonged unsymmetrically into two horns. The mesoblast ([fig. 160]) of the allantois would appear to grow round the whole inner surface of the subzonal membrane, but the cavity of the allantois only persists as a widish sack on the ventral aspect of the embryo (al). The amnion (am) is enormous, and is dorsally in apposition with, and apparently coalesces with the chorion, and ventrally covers the inner wall of the persistent allantoic sack. The chorion, except for a small area at the two poles and opposite the os uteri, is nearly uniformly covered with villi, which are more numerous than in [fig. 160]. In the large size of the amnion, and small dimensions of the persistent allantoic sack, the Cetacea differ considerably from the Ungulata.
Fig. 160. Diagram of the fœtal membranes in Orca gladiator. (From Turner.)
ch. chorion; am. amnion; al. allantois; E. embryo.
Manis. Manis amongst the Edentata presents a type of diffused placenta[92]. The villi are arranged in ridges which radiate from a non-villous longitudinal strip on the concave surface of the chorion.
Manis presents us with the third type of placenta found amongst the Edentata. On this subject, I may quote the following sentence from Turner (Journal of Anat. and Phys., vol. X., p. 706).
“The Armadilloes (Dasypus), according to Professor Owen, possess a single, thin, oblong, disc-shaped placenta; a specimen, probably Dasypus gymnurus, recently described by Kölliker[93], had a transversely oval placenta, which occupied the upper 2⁄3rds of the uterus. In Manis, as Dr Sharpey has shewn, the placenta is diffused over the surfaces of the chorion and uterine mucosa. In Myrmecophaga and Tamandua, as MM. Milne Edwards have pointed out, the placenta is set on the chorion in a dome-like manner. In the Sloths, as I have elsewhere described, the placenta is dome-like in its general form, and consists of a number of aggregated, discoid lobes. In Orycteropus, as I have now shewn, the placenta is broadly zonular.”