56 EXPERIMENTAL STUDIES ON FETAL ABSORPTION. 



the cat toward trypan-blue. For comparison a number of vitally stained pregnant 

 mice, rats, guinea-pigs, and rabbits were also examined. The dye was injected 

 into the mother intravenously on successive days. In the cats used, pregnancy was 

 more than half completed. 



In the gross the uterus of the vitally stained cat appeared deeply stained. On 

 opening it the placenta and unruptured membranes could be easily shelled out. The 

 placenta, which completely surrounds the fetus as a broad band or belt, was stained 

 a deep blue (fig. 13). The chorion (which incloses the poles of the fetus) was un- 

 stained with the exception of a zone a few millimeters in width, along the placental 

 borders, which was a mottled blue. This zone was found to be more marked late 

 than early in pregnancy. On rupturing the membranes the fetus, the umbilical 

 cord, the yolk-sac, and the allantoic and amniotic membranes were found entirely 

 unstained. There was no dye in the allantoic or the amniotic fluid. 



It may be said that in the cat. when the fetuses are normal and living, staining 

 of the amniotic fluid does not occur. In this my observation differs from that 

 originally made by Goldmann (1909), who described the amniotic fluid of the mouse 

 and rat as bright blue. It should be noted that in a later paper (1913) Goldmann 

 stated briefly, without giving further experimental data, that the amniotic, fluid, 

 as well as the embryo, is unstained. He had previously thought that his observa- 

 tion lent support to the theory that the amniotic fluid is in whole or in part derived 

 from the maternal blood-stream. From the present experiments it seems probable 

 that normally inert colloidal substances are unable to pass through the placenta or 

 outermost fetal membrane. These experiments shed no light on the derivation of 

 the amniotic fluid, and one can only venture the theory that the colloids normally 

 found in the amniotic fluid are probably not of direct maternal origin. In the 

 experiments described in the foregoing study trypan-blue was injected directly into 

 amniotic sacs of living fetuses, in which case there was rapid absorption of the dye 

 into the fetal blood-stream with vital staining of the embryo. 



Further evidence that trypan-blue does not ordinarily reach the blood-stream 

 of the fetus is afforded by the observation that several of the vitally stained cats 

 in these experiments gave birth to litters of living young which were unstained. 



Before describing the microscopic appearance of the vitally stained placenta 

 of the cat, it may be of interest to recall briefly its comparative development and 

 architecture. The placentae of mammalia have been classified by the degree of 

 union which occurs between the fetal and maternal tissues. Thus in the most 

 primitive type, seen in the pig, there exists merely an apposition of the chorionic 

 epithelium to the epithelium of the uterus. Consequently the maternal vessels 

 are widely separated from the fetal ones by a number of layers of cells. In the 

 slightly higher developed placenta' of ruminants the uterine epithelium is nearly 

 all absorbed by the trophoblast, so that a more intimate union of the chorion with 

 the maternal tissues results. In carnivora, further resorption of the maternal 

 tissues by the trophoblast occurs, so that finally a complex labyrinth of chorionic 

 villi and maternal vessels is formed. The maternal blood is separated from the 

 chorionic epithelium by a single layer of endothelial cells, and the distance from 

 maternal to fetal vessels is greatly diminished. Such a placenta has been termed 



