180 



by a syncytium. The syncytium forrns the lining of the large 

 iutervillous bloodspaces, and is contiuued over the embryonic 

 surface of the subepithelial tissue; isolated patches of it are found 

 included in the latter; it has probably a phagocytic action. 



It appears practically certain from the work of vau Heukelom 

 and Peters, that both layers are of trophoblastic origin ; and it 

 follows that the placenta arises in rnan, not by the penetration 

 of villi into maternal tissue, but by the formation of lacunae, 

 filled with extravasated maternal blood, in the trophoblast, and 

 the subsequent vaseularization of the latter by the capillaries of 

 the allantois. 



Carnivora. 



Duval's (7) very comprehensive account of the placentation of 

 the Dog and Cat constitutes, perhaps, the most important contri- 

 bution, that has yet been made to our knowledge of the placenta 

 of the Carnivora. Strahl (41, 42, 43, 46) has published a series 

 of papers dealing with the early stages in Dog, and Cat, Ferret, 

 Badger, Fox; while Heinricius (15, 16) has dealt with the Dog 

 and Cat, Fleischmann (9, 10, 11) with the Fox and Cat, and 

 Lüsebrink (26) with the Dog. 



In the Carnivora the yolk-sac is at first large, relatively to the 

 allantois, but is soon overtaken and outstripped by the latter, 

 which is a hollow sac of very considerable size extending on all 

 sides of the embryo. lts bloodvessels supply, it need not be 

 said, the placenta. The latter, as is well-knowu, is zonary, 

 being developed only from the middle, girdle-shaped area of the 

 trophoblast. 



We may begin with Duval's account of the placentation of 

 the Dog. 



At the period of heat a number of shallow crypts are formed 

 which open into the lumen of the uterus between the glands. 

 The latter are long and tortuous, and their deep ends are closely 

 applied against the muscularis. 



The changes that take place in the maternal tissues during 



