THE PLACENTA. 959 



shaped ; its vessels are merely arborescent, and do not penetrate the 

 chorion. Hence there is no special organ, or placenta^ intermediate be- 

 tween the embryo and the uterine walls, and these animals are there- 

 fore named Implacental Mammalia. 



In the porpoise, the pig, the horse, and, it is said, in the camel tribe 

 also, every part of the chorion and the allantoid endochorion, which 

 are coextensive, is covered with vascular processes ; these enter the 

 hypertrophied uterine membrane at all points, and form the so-called 

 diffused placenta. In the Ruminants, with the possible exception of 

 the camels, the vascular endochorion, or developed allantois, is also 

 coextensive with the chorion ; the vascular processes project, and at- 

 taching themselves to the uterus at certain definite scattered points of 

 the surface, form the embryonnl cotyledons ; these consist of clusters 

 of ramified villi, which fit into corresponding ramified canals, arranged 

 in cup-shaped depressions of the uterus, called the maternal cotyledons. 

 No decidua exists in these cases, between the embryonal and the ma- 

 ternal tissues, and they are easily detached from each other. The 

 blood in both is brought into close proximity, but the vessels of each 

 are independent, and the two bloods do not intermingle. In the dog, 

 cat, and Carnivora generally, the ovum, at first round, afterwards be- 

 comes oval, fusiform, or elongated, and occupies a compartment in the 

 elongated uterus. The endochorion, or allantoid sac, is very exten- 

 sive ; but the villous processes, absent from the ends of the ovum, form 

 a broad zone around its middle. A true decidua exists, and the com- 

 bined structures constitute the zonular placenta. In the Rodentia, 

 as is well seen in the rabbit, the allantoid sac reaches a small part 

 only of the inner surface of the chorion ; at this point alone perma- 

 nent vascular processes are developed, which, entering the hypertro- 

 phied uterine membrane, part of which forms a decidua, constitute a 

 discoid placenta. 



In the formation of the discoid human placenta, it is noticed that 

 the allantois is very small, appears early, and soon wastes ; it reaches 

 the chorion at only one point, not spreading out coextensively with it ; 

 and it conveys outwards, as usual, the umbilical arteries, the branches 

 of which enter the permanent villous processes of the shaggy chorion, 

 or foetal villi ; these are limited to one part of the ovum, penetrate the 

 decidual and hypertrophied portion of the uterine walls, and enter the 

 maternal venous sinuses. 



The two umbilical arteries and one umbilical vein are supported 

 upon the remains of the now impervious allantois, which grows into 

 a soft mucous connective tissue; with these parts also are found the 

 wasted vestiges of the vitelline duct, with its atrophied omphalo-mes- 

 enteric artery and veins ; these structures becoming elongated, and 

 surrounded with a tubular process of the amnion, form the umbilical 

 cord or navel string. This cord is, therefore, connected with the pla- 

 centa at one end, and with the navel of the embryo at the other. Its 

 vessels are always more or less spirally twisted. 



At the birth of the Mammalian embryo an event which, with the 

 human infant, happens at the end of the fortieth week the foetus -and 

 its membranes are detached from the inner surface of the uterus. The 



