Lemuridæ. The Lemurs in spite of their affinities with the Primates and Insectivora have, as has been shewn by Milne Edwards and Turner, an apparently very different form of placenta. There is only one embryo, which occupies the body and one of the cornua of the uterus. The yolk-sack disappears early, and the allantois (Turner) bulges out into a right and left lobe, which meet above the back of the embryo. The cavity of the allantois persists, and the mesoblast of the outer wall fuses with the subzonal membrane (the hypoblastic epithelium remaining distinct) to give rise to the chorion.
On the surface of the chorion are numerous vascular villi, which fit into uterine crypts. They are generally distributed, though absent at the two ends of the chorion and opposite the os uteri. Their distribution accords with Turner’s diffused type. Patches bare of villi correspond with smooth areas on the surface of the uterine mucosa in which numerous utricular glands open. There is no reflexa.
Although the Lemurian type of placenta undoubtedly differs from that of the Primates, it must be borne in mind that the placenta of the Primates may easily be conceived to be derived from a Lemurian form of placenta. It will be remembered that in Man, before the true placenta becomes developed, there is a condition with simple vascular villi scattered over the chorion. It seems very probable that this is a repetition of the condition of the placenta of the ancestors of the Primates which has probably been more or less retained by the Lemurs. It was mentioned above that the resemblance between the metadiscoidal placenta of Man and that of the Cheiroptera, Insectivora and Rodentia is rather physiological than morphological.
Comparative histology of the Placenta.
It does not fall within the province of this work to treat from a histological standpoint the changes which take place in the uterine walls during pregnancy. It will, however, be convenient to place before the reader a short statement of the relations between the maternal and fœtal tissues in the different varieties of placenta. This subject has been admirably dealt with by Turner (No. [222]), from whose paper [fig. 161] illustrating this subject is taken.
The simplest known condition of the placenta is that found in the pig (B). The papilla-like fœtal villi fit into the maternal crypts. The villi (v) are formed of a connective tissue cone with capillaries, and are covered by a layer of very flat epithelium (e) derived from the subzonal membrane. The maternal crypts are lined by the uterine epithelium (e´), immediately below which is a capillary flexus. The maternal and fœtal vessels are here separated by a double epithelial layer. The same general arrangement holds good in the diffused placentæ of other forms, and in the polycotyledonary placenta of the Ruminantia, but the fœtal villi (C) in the latter acquire an arborescent form. The maternal vessels retain the form of capillaries.
In the deciduate placenta a considerably more complicated arrangement is usually found. In the typical zonary placenta of the fox and cat (D and E), the maternal tissue is broken up into a complete trabecular mesh-work, and in the interior of the trabeculæ there run dilated maternal capillaries (d´). The trabeculæ are covered by a more or less columnar uterine epithelium (e´), and are in contact on every side with fœtal villi. The capillaries of the fœtal villi preserve their normal size, and the villi are covered by a flat epithelial layer (e).
In the sloth (F) the maternal capillaries become still more dilated, and the epithelium covering them is formed of very flat polygonal cells.
