MAMMALIA. 215 



A. Placenta in its most generalized form . 



B. Structnre of placenta of a Pig. 



C. Structure of placenta of a Cow. 



D. Structure of i^lacenta 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, 

 dii, dn. represents the decidua serotina of the placenta ; t, t. trabeculse of serotina 

 passing to the foetal villi ; ca. curling artery ; up. utero-placental vein; ,t. a prolonga- 

 tion of maternal tissue on the exterior of the villus outside the cellular layer e', which 

 may represent either the endothelium of the maternal blood-vessel or delicate con- 

 nective 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 i^lacenta. 



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 Ijorne 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 deve- 

 loped, 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 morpho- 

 logical. 



Comparative histology of the Place)da. 



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 pregnaiicy. It will, however, be convenient to place before the 

 reader a short statement of the relations between the maternal and 

 fojtal 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 foetal villi fit into the maternal crypts. The 

 villi iy) 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 foetal 

 vessels are here separated by a double epithelial layer. The same general 

 arrangement holds good in the diffused placentae of other forms, and in 

 the polycotyledonary placenta of the Ruminantia, but the foetal villi (0) 

 in the latter acquire an arborescent form. The maternal vessels retain 

 the form of capillaries. 



In the deciduate placenta a considerably more complicated arrange- 

 n\ent is usually found. In the typical zonary placenta of the fox and cat 

 (D and E), the maternal tissue is broken up ijito a complete trabecular 



