462 OP GENERATION AND DEVELOPMENT. 



Urachus or suspensory ligament of the latter represents the duct by 

 which the Allantois was originally connected with the abdominal cavity. 



818. The connexion which is thus formed between the Vascular sys- 

 tem of the foetus and that of the parent, is the only one that exists in 

 the lower Mammalia ; which are thus properly designated as"nonpla- 

 cental." Each villus of the Chorion contains a capillary loop ; this 

 is enclosed in a layer of cells ; and this again in a lamina of basement- 

 membrane ; the whole forming the foetal tuft. This comes into contact 

 with the cellular decidua, which lies upon the basement-membrane 

 covering the vascular layer of the decidua. Now the Placenta is com- 

 posed of these very elements, arranged in a more complex manner. It 

 is formed by an extension of the vascular tufts of the Chorion at certain 

 parts ; and a corresponding adaptation, on the part of the Uterine struc- 

 ture, to afford to these an increased supply of nutritious fluid. These 

 specially-prolonged portions are scattered, in the Ruminants and some 

 other Mammalia, over the whole surface of the Chorion, forming what 

 are termed the Cotyledons ; but in the higher orders, and in Man, they 

 are concentrated in one spot, forming the Placenta. In some of the 

 lower tribes, the maternal and foetal portions of the placenta may be 

 very easily separated ; the former consisting of the thickened decidua ; 

 and the latter being composed of the prolonged and ramifying vascu- 

 lar tufts of the Chorion, which dip down into it. But in the Human 

 placenta, the two elements are mingled together through its whole 

 substance. 



819. The foetal portion of the Placenta consists of the branches of 

 the umbilical vessels ; which subdivide at the point at which they enter 

 the mass, and form, by their minute ramifications, a large part of its 

 substance. Each villus contains a capillary vessel, which forms a 

 series of loops, communicating with an artery on the one side, and with 

 a vein on the other; but the same capillary may enter several villi, 

 before re-entering a larger vessel. The vessels of the villi (Fig. 147, g} 

 are covered, as in the chorion, by a layer of cells (/), enclosed in base- 

 ment-membrane (e); but the foetal tuft thus formed is enclosed in a 

 second series of envelopes (a, 5, c\ derived from the maternal portion 

 of the placenta ; a space (d) being left between the two, however, at 

 the extremity of the tuft. The vascular tufts not unfrequently extend 

 beyond the uterine surface of the placenta, and dip down into the ute- 

 rine sinuses, where they are bathed in the maternal blood. The ma- 

 ternal portion of the Placenta may be regarded as a large sac, formed 

 by a prolongation of the internal coat of the great uterine vessels. 

 Against the foetal surface of this sac, the tufts just described may be 

 said to push themselves, so as to dip down into it, carrying before 

 them a portion of its thin wall, which constitutes a sheath to each tuft. 

 In this manner, the whole interior of the placental cavity, is intersected 

 by numerous tufts of foetal vessels, disposed in fringes, and bound down 

 by the membrane that forms its proper wall ; just as the intestines are 

 covered and held in their places by the peritoneum. Now as this dila- 

 tation of the uterine blood-vessels carries the decidua before it, every 

 one of the vascular tufts that dips down into it will be covered with a 



