92 EXPERIMENTAL STUDIES ON FETAL ABSORPTION. 



be derived from the neighboring vitelline membrane and this, he thought, was an 

 argument in favor of the origin of the amniotic fluid, in part at least, from the 

 maternal blood. It is more probable that the trypan blue occasionally seen in 

 the amniotic fluid of rodents first enters the fetal blood-stream through both the 

 vitelline membrane and the placenta. A trace of the dye in the fetal blood might 

 readily escape detection, whereas, as soon as it had diffused from there into the 

 colorless amniotic fluid it would attract attention. That it immediately enters 

 the amnion from the vitelline membrane, without first entering the fetal circulation, 

 seems improbable, for the vitelline membrane is well vascularized by the omphalo- 

 mesenteric vessels and, should a dye diffuse from its epithelium through the base- 

 ment membrane, it would necessarily be swept into these large vascular channels 

 before it could enter the amnion. 



The outside of the placenta appeared blue and on gross section the entire 

 organ was found similarly stained with the exception of an area about 5 by 3 mm., 

 which occupied a central position between the placental labyrinth and decidua (fig. 1) . 

 As will be shown later, this white tissue consists of chorionic villi possessing a peculiar 

 arrangement and designated by Duval as "the roof of the central excavation." 



In many rodents, such as the rat, mouse, and guinea-pig, the vitelline mem- 

 brane, as described by Duval and Sobotta, becomes the outermost fetal covering 

 and persists throughout intrauterine life. Its epithelial surface comes in direct 

 contact with the uterine wall and serves to nourish the fetus by the absorption of 

 embryotrophe and the assimilation of extra vasated maternal blood. This is more 

 readily accomplished by the development of numerous villi which project into the 

 uterine space and are covered by a single layer of cylindrical, phagocytic epithelial 

 cells. This layer stains very rapidly and deeply with trypan blue (figs. 1 and 2). 

 A single injection suffices, after a few hours, to cause the cytoplasm of the cells to 

 become heavily laden with fine blue granules. One is justified in speaking of the 

 cells covering the villi as an attraction center for vital dyes, for nowhere else in the 

 body are the dye-stuffs so rapidly taken up. 



The villi show their greatest development near the line of attachment to the 

 placenta, while towards the equator of the vitelline membrane they become 

 progressively lower. Finally they disappear, so that the membrane at its anti- 

 mesometral pole is perfectly smooth. During this transition the epithelium changes 

 from a cylindrical to a cuboidal type. The latter is probably less phagocytic than 

 the former, for the cuboidal cells contain relatively few trypan-blue granules. 



The villi cease suddenly near the attachment of the vitelline membrane to the 

 placental surface (figs. 1, 2), and the cylindrical, vitally staining epithelium changes 

 abruptly to a delicate, flattened type which fails to absorb the dye. This un- 

 stained, flattened tissue completely lines the angle (sinus entodermaticus) formed 

 by the membrane at its attachment to the placenta. As the vitelline membrane 

 (ectoplacental endoderm) spreads out laterally over the surface of the placenta, its 

 epithelium again undergoes a change (fig. 2). The cells proliferate, forming in 

 many places small tufts or clusters, the older cells of which gradually become 

 constricted off by new cells arising from the basement membrane. 



