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CHAPTER XLIII. 



OF THE MEMBRANES OF THE FCETUS. OF THE STRUCTURE OF THE 



CHORION. OF THE AMNION. LIQUOR AMNII. OF THE UMBI- 

 LICAL VESICLE. OF THE ALLANTOIS. ALLANTOIC FLUID. UM- 

 BILICAL CORD. BIRTH. 



Formation of the Placenta. The early development of the 

 chorion has been described in a former page. At first, the villi are 

 composed entirely of cells, invested on their external surface with 

 a very delicate structureless membrane ; but after the vessels, con- 

 ducted by the allantois, have reached its inner surface, vascular 

 loops are prolonged into them. Bischoff considers that in the 

 human ovum, and in that of the bitch, which are destitute of an 

 albuminous covering, the tufts are formed directly from the zona 

 pellucida alone. 



In two orders of mammalia, the marsupialia and the monotremata, there is 

 no connection between the vascular system of the mother and that of the 

 foetus, which is nourished from a very early period with milk. 



The relation between the blood of the foetus and that of the mother is 

 nearly the same in placental mammalia. The wall of the maternal ves- 

 sels, a layer composed of cells from the modified mucous membrane of the 

 uterus, and another cellular layer belonging to the foetal tuft, always separate 

 them ; but in the greater number of mammalia, the foetal tufts come into 

 relation with the capillary vessels of the mother ; while in man they are in 

 contact with the walls of & large cavity containing blood. 



The mode of arrangement of the tufts, or, in other words, the form which 

 the placenta assumes, is very different in the various mammalia. Sometimes 

 the whole chorion is covered with villi, as in the pachydermata (hog, ele- 

 phant, etc.) ; sometimes these form little collections or cotyledons, as in the 

 greater number of ruminants (sheep, ox, goat, etc.) ; sometimes they form a 

 band encircling the central portion of the chorion, as in the carnivora ; and 

 in some instances they are confined to one single part, forming a single pla- 

 centa, as in the rodentia, and also in the human subject (vide figs. 280, 292). 



The beautiful branched and highly complicated conical foetal villi of the 

 ruminant dip into deep recesses in the maternal cotyledons, upon the walls of 



