124 COMPARATiyE PHYSIOLOGY. 



identical in minute structure in most instances. In each case 

 the blood-vessels are covered superficially by cells which we 

 can not help thinking are essential in nutrition. The villi are 

 both nutritive and respiratory. It is no more difficult to under- 

 stand their function than that of the cells of the endoderm of a 

 polyp, or the epithelial coverings of lungs or gills. 



Experiment proves that there is a respiratory interchange 

 of gases between the maternal and foetal blood which nowhere 

 mingle physically. The same law holds in the respiration of 

 the foetus as in the mammals. Oxygen passes to the region 

 where there is least of it, and likewise carbonic anhydride. If 

 the raother be asphyxiated so is the foetus, and indeed more 

 rapidly than if its own umbilical vessels be tied, for the mater- 

 nal blood in the first instance abstracts the oxygen from that 

 of the foetus when the tension of this gas becomes lower in the 

 maternal than in the foetal blood ; the usual course of afFairs 

 is reversed, and the mother satisfies the oxygen hunger of her 

 own blood and tissues by withdrawing that which she recently 

 supplied to the foetus. It will be seen, then, that the emliryo is 

 from the first a parasite. This explains that exhaustion which 

 pregnancy, and especially a series of gestations, entails. True, 

 nature usually for the time meets the demand by an excess of 

 nutritive energy : hence many animals are never so vigorous in 

 appearance as when in this condition ; often, however, to be fol- 

 lowed by corresponding emaciation and senescence. The full 

 and frequent respirations, the bounding pulse, are succeededlby 

 reverse conditions ; action and reaction are alike present in the 

 animate and inanimate worlds. Moreover, it falls to the parent 

 to eliminate not only the waste of its own organism but that of 

 the foetus ; and not infrequently in the human subject the over- 

 wrought excretory organs, especially the kidneys, fail, entailing 

 disastrous consequences. 



The digestive functions of the embryo are naturally inact- 

 ive, the blood being supplied with all its needful constituents 

 through the placenta by a much shorter process ; indeed, the 

 placental nutritive functions, so far as the foetus is concerned, 

 may be compared with the removal of already digested ma- 

 terial from the alimentary canal, though of course only in a 

 general way. During foetal life the digestive glands are 

 developing, and at the time of birth all the digestive juices 

 are secreted in an efficient condition, though only, relatively 

 so, necessitating a special liquid food (milk) in a form in which 



