92 



COMPARATIVE PHYSIOLOGY. 



In the placenta of the 

 apes and of the human sub- 

 ject the most marked depart- 

 ure from simplicity is found. 

 The maternal vessels are said 

 to constitute large intercom- 

 municating sinuses ; the villi 

 may hang freely suspended in 

 these sinuses, or be anchored 

 to their walls by strands of 

 tissue. There is believed to 

 be only one layer of epithe- 

 lial cells hetween the vessels 

 of mother and foetus in the 

 later stages of pregnancy. 

 This, while closely investing 



Pig. 96.— Placenta of a sloth. Flat maternal the foetal vessels (capillaries), 

 epithelial cells Bhown in position on -,-, t ■, , .i - , 



right side; on left they are removed and really belongs to the mater- 



dUated; maternal vessel with its blood- „„i „i„,_i_,„„_ rri,„ _;_„;« 



corpuscles exposed. nal Structures, ihe signifl- 



6. p. 



Fig. 97.— Structure of human placenta; ds, decidua serotina; t, trabeculse of serotlna 

 passing to foetal villi; ca, curling artery; up, ntero-placental vein; x, prolongation 

 of maternal tissue on exterior of villus, outside cellular layer e', wbicn may repre- 

 sent either endothelium of maternal blood-vessels or delicate connective tissue of 

 the serotina or both; e', maternal cells of the serotina. 



