980 THE PHYSIOLOGY OF REPRODUCTION. 



the fetal blood-vessels and the epithelial layers of the chorionic villi, 

 but an active diffusion relation is set up between them. Nutritive 

 material, protein, fat, and carbohydrate, and oxygen pass from 

 the maternal to the fetal blood, and the waste products of fetal 

 metabolism carbon dioxid, nitrogenous wastes, etc., pass from the 

 fetal to the maternal blood. The nutrition of the fetal tissues is 

 maintained, in fact, in much the same way as though it were an 

 actual part of the maternal organism. That material passes from 

 the maternal to the fetal blood is a necessary inference from the 

 growth of the fetus. The fact has also been demonstrated repeat- 

 edly by direct experiment. Madder added to the food of the mother 

 colors the bones of the embryo. Salts of various kinds, sugar, drugs, 

 etc., injected into the maternal circulation may afterward be de- 

 tected in the fetal blood. But we are far from having data that 

 would justify us in supposing that the exchange between the* 

 two bloods is effected by the known physical processes of os- 

 mosis, diffusion, and filtration. The difficulties in understanding 

 the exchange in this case are the same as in the absorption of nour- 

 ishment by the tissues generally. It is perhaps generally assumed 

 that the chorionic villi play an active part in the process, func- 

 tioning, in fact, in much the same way as the intestinal villi. This 

 assumption implies that the epithelial cells of the villi take an active 

 part in the absorption of material by virtue of processes which can- 

 not be wholly explained, but which without doubt are due to the 

 chemical and physical properties of the substance of which they are 

 composed. This assumption does not -mean that the simpler 

 and better understood physical properties of diffusion and osmosis 

 are not also important. The respiratory exchange of gases, the 

 diffusion of water, salts, and sugar, may be largely controlled in this 

 way. There are no facts at least which contradict such an assump- 

 tion. The passage of fats and proteins, however, would seem to 

 require some special activity in the chorionic tissue, which may be 

 connected with the presence of special enzymes. Glycogen occurs 

 in the placenta itself and in all the tissues of the embryo during the 

 period of most active growth. In the later period of embryonic 

 life, as the liver assumes its functions, the glycogen becomes 

 more localized to this organ and disappears, except for traces, 

 in the skin, lungs, and other tissues in which it was present at 

 first in considerable quantities. It would appear, therefore, that 

 glycogen (sugar) represents one of the important materials for the 

 growth of the embryo, and that in the beginning at least the tissues 

 generally have a glycogenetic power. The sugar brought to the 

 placenta in the maternal blood passes over into the fetal blood and 

 the excess beyond that immediately consumed is deposited in the 

 tissues as glycogen. The body fat of the fetus is at first slight in 



