EXPERIMENTAL STUDIES ON FETAL ABSORPTION. 99 



man, the highest types, a still more intimate union of fetal and maternal tissues 

 occurs. Here the maternal blood eventually flows in channels completely lined by 

 fetal ectoderm, and hence only a layer of syncytium and delicate connective tissue 

 separates the maternal blood from the fetal capillaries. It is apparent that in these 

 animals (rabbit, guinea-pig, mouse, rat, and man) in which tetanus antitoxin or 

 traces of trypan blue have been found in the fetus or the amniotic fluid, an extremely 

 thin barrier of cells separates the maternal from the fetal blood-stream. In the 

 cat, sheep, and cow, on the other hand, where the union is less intimate and numerous 

 cells intervene between the two circulations, the transmission of antitoxin and trypan 

 blue has not been observed. 



The vitelline membrane of the rabbit is composed of a single layer of columnar 

 cells, resting on a basement membrane beneath which are numerous blood-vessels. 

 The surface of the membrane at its antimesometral pole is smooth, while in the 

 neighborhood of the terminal sinus, where it ends in a ragged edge, it is covered by 

 villi which project into the semi-fluid embryotrophe imprisoned between them and 

 the uterine wall. In the injected animal the vitelline membrane is deep blue on 

 gross appearance, and microscopically granules of trypan blue are found in large 

 numbers in the columnar cells composing the membrane (fig. 8). These large 

 granules, usually 20 or more to a cell, are scattered throughout the cytoplasm. The 

 single oval or round nucleus contains none of the dye. No granules are visible in the 

 basement membrane or beneath it in the walls of the fetal vessels. The amniotio 

 membrane contains no trace of dye. 



In the rabbit the labyrinth comprises the bulk of the placenta during the second 

 half of pregnancy. Septa of fetal connective tissue, containing branches of the 

 umbilical vessels, divide it roughly into lobules. The umbilical vessels break up 

 within the lobules into capillaries which are completely lined by endothelial cells 

 and are accompanied by delicate supporting strands of mesoderm. The space 

 between the capillaries is occupied by a syncytium composed of fetal ectoderm, the 

 meshes of which surround innumerable tiny spaces in which the maternal blood 

 circulates. In consequence of this arrangement the maternal and fetal blood- 

 streams are separated by only a thin layer of syncytium and the delicate endo- 

 thelium of the fetal capillaries. In fact, in many places even the syncytium 

 appears to be absent, so that the maternal blood-cells come in direct contact with 

 the fetal endothelium, an arrangement no doubt greatly facilitating the interchange 

 of substances. 



Trypan blue was found abundantly present in the placental labyrinth as 

 aggregations of tiny granules throughout the syncytium (fig. 9). None of it was 

 seen in the cells forming the fetal capillaries. Beneath the labyrinth, and uniting 

 it to the uterine wall, lies a layer of giant cells. These cells, which early in gestation 

 play a prominent part in the growth of the placenta and the nutrition of the fetus, 

 are reduced during the latter half of gestation to a relatively inconspicuous layer. 

 In these vitally stained rabbits the cytoplasm of these cells became filled with 

 numerous tiny blue granules, showing that the cells, even with approaching 

 maturity, possess the power to assimilate materials brought to them. 



