CHANGES IN THE OVUM. 113 



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. The 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 foetus itself. It is certain, however, that the 

 placental absorption is by far the most important, and that the rapid 

 development of the fcetus is mainly, if not altogether, due to the nutritive 

 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 remembered that it 

 does contain much of the nutritious elements, and that the foetus, which 

 is unprovided with a mouth, and therefore cannot swallow, is neverthe- 

 less as well developed as one that does ingest this fluid. 



Nutritioji. 



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 placentulae. There the mater- 

 nal blood is conveyed by certain arteries into particular sinuses or recep- 

 tacles 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 bathed in the blood of the parent, as 

 the " gills " or branchiae 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 foetal blood to become arterial, by en- 

 abling it to part with its carbonic acid, and also to get rid of excrementi- 

 tious matter derived from the different processes connected with the 

 growth of the foetus, and whose retention in the blood of the young crea- 

 ture 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 nu- 

 tritious elements of the maternal blood — its fluid portion only — to be 

 taken up into the system of the foetus. 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 

 prepared by two sets of cells, one of which — the maternal — transmits it to 

 the other — the fcetal — 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 assimilated by the cells which float in the circula- 

 ting fluid. No other communication between the two vascular systems 

 exists ; and the fact that the blood corpuscles of each are diflferent 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 offspring, renders it evident 

 that the maternal blood may become impregnated with substances, or 

 impressed with attributes, which will affect the development or modify the 

 constitution of the fcetus ; while pernicious matters generated in the latter, 

 may prove more or less noxious to the mother. 



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