MAMMALIA. 257 



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 deve- 

 loped, there is a condition with simple vascular villi scattered over the cho- 

 rion. 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, In- 

 sectivora 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 histologi- 

 cal 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 fetal 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 fcetal 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 fcetal 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 polycotyledo- 

 nary placenta of the Ruminantia, but the fcetal 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 meshwork, 

 and in the interior of the trabeculae there run dilated maternal capillaries 

 (</). The trabeculae are covered by a more or less columnar uterine epithe- 

 lium (<?'), and are in contact on every side with fcetal villi. The capillaries of 

 the fcetal 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. 



In the human placenta (G), as in that of Apes, the greatest modification 



B. III. 17 



