THE FEMALE REPRODUCTIVE ORGANS. 989 



villi, but an active diffusion relation is set up between them. 

 Nutritive materials, protein, 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 detected in the fetal blood. It is a question as to 

 how far this transferral is effected through the placenta by phys- 

 ical diffusion, and what especial part, if any, the placenta takes 

 in elaborating or secreting the material. Most of the evidence at 

 hand indicates that the passage from the mother's blood to the 

 fetal blood, and vice versa, is chiefly a process of diffusion.* The 

 nitrogenous waste products in the fetal blood, the urea, creatinin, 

 creatin, uric acid, and ammonia, have the same concentrations in 

 the two bloods, and this fact would indicate that these substances 

 are in free diffusion relations through the placenta. So, too, the 

 sugar has the same concentration in the two bloods, and we may 

 assume that this material diffuses easily from mother to fetus, 

 giving to the latter its supply of carbohydrate food. 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 protein food according 

 to our present ideas is carried in the blood in the form of amino- 

 acids. Examination shows that the fetal blood contains slightly 

 more than the fetal blood. In this case, therefore, in addition to 

 the diffusion that undoubtedly may take place, there may be a 

 special absorptive power on the part of the placental tissue by 

 means of which a somewhat higher equilibrium in concentration 

 is maintained in the fetal blood. In the matter of fat transport 

 through the placenta experiments with vital staining of the fat 

 * Siemens, "The Nutrition of the Fetus," New Haven, 1919. 



