HUMAN FETAL MEMBRANES. PLACENTA. FETAL CIRCULATION. 977 



that pass to the decidua reflexa. Between the chorion and the amnion there is a 

 gelatinous layer (membrana intermedia) of immature connective tissue. 



The large villi of the chorion frondosum (Fig. 379) penetrate more 

 deeply into the uterine mucosa and first of all into the ducts of the glands, 

 like roots into loose soil. According to Selenka this penetration takes 

 place from the first week. From the syncytial covering of the villi 

 there arise in especially large number between the second and the third 

 month cell-buds, which are probably related to the nourishment of 

 the embryo. In the further penetration of the villi through the ducts 

 of the glands they make their way through the walls of the large contigu- 

 ous blood-vessels, which in structure are similar to the capillaries, so 

 that the villi, bathed in the maternal blood (uterine vessels), float in 

 these enormous decidual capillaries the so-called intervillous spaces 

 (Fig. 376, VIII, b). The villi within the blood-spaces are covered by 

 the epithelium of the latter. 



Some villi that have no epithelium unite by means of bulbous ex- 

 tremities with the tissue of the uterine placenta and thus form adherent 

 villi ; a means of firm union. In 

 this way the 'placenta is formed; 



Q HictinrH-irvn ic rnarlp "hpl-wppn thp btrorna and Lateral buds Epithelium of 



^LWCt capillaries of O f the villi. the villi. 



fetal placenta, which includes the 

 entire mass of villi, and the uter- 

 ine or maternal placenta, the por- 

 tion of the endometrium in 

 relation with the ovum, which 

 is especially rich in vessels at 

 this point. The two parts are 

 not separable even at the time of 

 birth. Venous maternal vessels 

 of considerable size course around 

 the border of the placenta, con- 

 stituting the marginal sinus. The Vascular tnmi 

 placenta is the nutritive and res- 



, 1 , 1 1 FlO. 370- Isolated Portion of Villi from a Human 



piratory organ of the fetus, which Placenta, 



obtains the necessary material 

 through endosmosis from the ma- 

 ternal blood-spaces through the coverings and vessel-walls of the villi, in 

 which the fetal blood circulates. 



Between the placental villi there is a clear fluid that contains numerous small, 

 albuminoid globules and is designated uterine milk (abundant in the cow) ; it is 

 believed to originate from degeneration of decidual cells. It is thought, together 

 with the blood, to take part in the process of nutrition. 



The investigations of Walter have shown that when pregnant animals are 

 poisoned with strychnin, morphin, veratrin, curare, and ergotin, these substances 

 cannot be demonstrated in the fetus. Certain other chemical substances, for 

 example phosphorus, potassium chlorate, potassium bromid, potassium iodid, 

 arsenic, mercury, alcohol, phenol, morphin, and methylene-blue do, however, 

 pass over to the fetus. Some substances pass also from the fetus to the maternal 

 body. An examination of the placenta shows that its villi are grouped in in- 

 dividual sections of considerable size, between which are furrow-like indentations. 

 These individual complexes may be compared with the cotyledons of lower 

 animals. 



The position of the placenta is. as a rule, upon the anterior or posterior uterine 

 wall, less commonly at the fundus uteri, or laterally in front of or beneath 

 a tubal opening (lateral placenta) or in front of the internal orifice of the uterus 

 62 



