PHYSIOLOGICAL 111 ES >>. MESA IS THK ElETIS. 121 



embryo. And, finally, when the chorion is at last organised, and 

 furnished with its myriads of vascular papilhe in the form of dissemi- 

 nated or a^'glomerated placentuhv, absorption goes on with exaggerated 

 activity over the whole uterine surface and texture, in order to supply 

 all the nutrition required for the now rapid development of the foetus. 



The nutritive elements are absorbed by the vessels of the placental 

 villosities, transformed into blood, and conveyed to the young creature 

 by the umbilical veins. This extra-fcetal absorption is supplemented by 

 that which is taking place in the fluid of the envelopes, and also in the 

 organs and tissues of the fa>tus itself. It is certain, however, that the 

 l)lacental absorption is by far the most important, and that the rapid 

 development of the fa^tus is mainly, if not altogether, due to the nutri- 

 tive elements obtained there ; while it is not improbable that the 

 amniotic fluid found in the stomach and intestines may act as a dilator 

 of these, modify the action of the bile accumulated in the latter, and 

 perhaps yield a small amount of nutriment ; though it must be'remem- 

 bered that it does contain much of the nutritious elements, and that 

 the fa?tus which is unprovided with a mouth, and therefore cannot 

 swallow, is nevertheless as well developed as one that does ingest this 

 fluid. 



Nutrition. 



It has been shown that the foetal vascular system is quite distinct 

 from that of the mother, the isolation of the two systems being complete, 

 and only brought into contact at the placenta or placentuhc. There the 

 maternal blood is conveyed by certain arteries into particular sinuses 

 or receptacles of the uterus containing the ultimate radicles of these 

 vessels, which emerge into veins ; while the foetal vessels, extremely 

 attenuated, dip down into these receptacles and are batlied in the 

 blood of the parent, as the "gills" or branchiic of aquatic creatures 

 are in the water in which they live. But the actual blood of parent 

 and offspring never meet : they are only brought into indirect contact; 

 and between them are the thin coats of vessels, basement membranes, 

 and cells. This indirect contact is sufficient to permit the venous fcctal 

 blood to become arterial, by enabling it to part with its carbonic acid, 

 and also to get rid of excrementitious matter derived from the different 

 processes connected with the growth of the foetus, and whose retention 

 in the blood of the young creature would doubtless be a source of injury 

 to it. This arrangement of the two systems of vessels also, as has just 

 been said, allows certain nutritious elements of the maternal blood — its 

 fluid portion only — to be taken up into the system of the fu-tus. In 

 this way, as has been pointed out, these vascular rootlets of the placenta 

 closely correspond to the villi of the mucous membrane of the intestines; 

 and the analogy is rendered more complete when we know that the 

 nutrient material is selected and prepb,red by two sets of cells, one of 

 which — the maternal — transmits it to the other — the fcttal — in the 

 same manner as the epithelial cells of the intestinal villi seem to take 

 up and prepare the nutrient matter that is destined to be again assimi- 

 lated by the cells which float in the circulating fluid. No other com- 

 munication between two vascular systems exists ; and the fact that the 

 blood corpuscles of each are different in size, conclusively demonstrates 

 that they must be distinct. Nevertheless, the special function of the 

 placenta, and the intimate relation existing, through it, between the 

 fluids of parent and ofl'spring, renders it evident that the maternal 



