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 foetal villi. The capillaries of the foetal 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. [FIG. 161. DIAGRAMMATIC REPRESENTATIONS OF THE MINUTE STRUCTURE OF THE PLACENTA. (From Turner.) _F._ the foetal; _M._ the maternal placenta; _e._ epithelium of chorion; _e´._ epithelium of maternal placenta; _d._ foetal blood-vessels; _d´._ maternal blood-vessels; _v._ villus. A. Placenta in its most generalized form. B. Structure of placenta of a Pig. C. Structure of placenta of a Cow. D. Structure of placenta of a Fox. E. Structure of placenta of a Cat. F. Structure of placenta of a Sloth. On the right side of the figure the flat maternal epithelial cells are shewn _in situ_. On the left side they are removed, and the dilated maternal vessel with its blood-corpuscles is exposed. G. Structure of Human placenta. In addition to the letters already referred to _ds_, _ds_. represents the decidua serotina of the placenta; _t_, _t_. trabeculæ of serotina passing to the foetal villi; _ca._ curling artery; _up._ utero-placental vein; _x._ a prolongation of maternal tissue on the exterior of th villus outside the cellular layer _e´_, which may represent either the endothelium of the maternal blood-vessel or delicate connective tissue belonging to the serotina, or both. The layer _e´_ represents maternal cells derived from the serotina. The layer of foetal epithelium cannot be seen on the villi of the fully-formed human placenta.] In the human placenta (G), as in that of Apes, the greatest modification is found in that the maternal vessels have completely lost their capillary form, and have become expanded into large freely communicating sinuses (_d´_). In these sinuses the foetal villi hang for the most part freely, though occasionally attached to their walls (_t_). In the late stages of foetal life there is only one epithelial layer (_e´_) between the maternal and foetal vessels, which closely invests the foetal villi, but, as shewn by Turner and Ercolani, is part of the uterine tissue. In the foetal villi the vessels retain their capillary form. _Evolution of the Placenta._