958 SPECIAL PHYSIOLOGY. 



so-called allantoid sac. . This outer part extends itself till it reaches 

 the inner surface of the chorion. As already described, this last-named 

 structure is formed upon the altered vitelline membrane, and soon be- 

 comes covered on its outer surface with little knobs ; these are devel- 

 oped into temporary villous processes, composed entirely of nucleated 

 cells, and employed in absorbing nourishment for the early mammalian 

 embryo. After a time these simple villi disappear, being, as it were, 

 obliterated by the growth and distension of the chorion. But then, 

 the chorion itself, and the outer layer of the amnion, or false amnion, 

 which is now in close relation with the chorion, becomes the seat of 

 development of other processes, which, with a thin covering of cells 

 from the chorion itself, form the ramified tufted villi of the so-called 

 shaggy chorion. As the allantoid sac of the Mammalia, with its con- 

 tained bloodvessels, grows, it reaches the inner surface of the chorion, 

 and its vessels entering the villous processes of the latter, form loops 

 in their interior. These processes, now vascular, and constituting the 

 embryonal or foetal portion of the placenta, penetrate through the de- 

 cidua into the maternal portion of the placenta, projecting into its ve- 

 nous sinuses and lacunas, and form the so-called foetal villi. These are 

 covered by their own epithelium and basement membrane, and also by 

 a loose layer belonging to the lining-membrane of the maternal venous 

 sinuses. The blood of the Mammalian embryo, passing along the um- 

 bilical arteries, upon the allantois, circulates through these foetal villi, 

 which are themselves bathed with the maternal blood. The two bloods 

 come into close relation, being separated only by the most delicate 

 tissues, but they do not intermingle. In this way nutriment is ab- 

 sorbed from the maternal blood, for the maintenance of the growth of 

 the embryo ; and possibly eifete matters are especially eliminated from 

 the embryonal blood. This latter blood is oxygenated by a respiratory 

 process, consisting of an interchange of carbonic acid and oxygen be- 

 tween the embryonal and maternal blood, just as occurs in the gills of 

 the Amphibian tadpole, and of the Fish, which are bathed in water. 

 The blood in the umbilical arteries of the embryo is, as we shall see, 

 nearly all dark or venous blood ; that in the maternal venous sinuses 

 is really arterial, for the maternal portion of the placenta contains no 

 capillaries, the branches of the uterine arteries which enter it termi- 

 nating at once in the venous lacunae, from which the true uterine veins 

 pass obliquely. Having been properly purified and nourished, the 

 embryonal blood returns from the placenta, enters the umbilical veins, 

 arid through them, reaches the embryo again. In the Reptile and 

 Bird the respiration of the embryo takes place between the embryonal 

 blood in the vessels of the allantois, and the atmospheric air in the 

 fluids of the egg, or outside the shell-membrane ; in neither Class do 

 the vessels of the allantois or the branches of the umbillical vessels 

 penetrate the outer coverings of the ovum, as occurs in the Mammalia 

 generally. 



The exact relations of the allantoid sac and its vessels with the 

 chorion, and especially the extent to which it covers the interior of 

 that coat, vary in the different Orders of Mammalia. In the Mono- 

 tremata and Marsupialia, for example, the allantois is small and pear- 



