EXPERIMENTAL STUDIES ON FETAL ABSORPTION. 93 



Duval noted the remarkable papillary hyperplasia of the epithelium in this 

 region but did not ascribe to it any function. These cells do not stain vitally, as do 

 those of the neighboring villi, and therefore their function possibly is not one of 

 phagocytosis or absorption. It is interesting to note that the greatest proliferation 

 of the epithelium occurs during the middle of pregnancy and that towards term it 

 ceases. At birth the vitelline membrane in this region has been reduced to a single 

 layer of columnar or cuboidal epithelial cells resting upon a simple basement 

 membrane. 



We shall now consider the vitally stained placenta. From a knowledge of the 

 behavior of the placenta of the mouse and rat towards trypan blue, we should 

 expect to find the dye abundantly present in the chorionic epithelium. We should 

 look for it particularly in the giant cells and syncytium of fetal origin which invade 

 and destroy the maternal tissue and attach the placenta to the uterine wall, and also 

 in the delicate epithelial syncytium of the labyrinth which covers the villi and forms 

 a barrier between the maternal blood-stream and the fetal capillaries. 



To the unaided eye the placenta appears quite blue, with the exception of a 

 tiny flattened area near its center, which always remains unstained (figs. 1, 4). On 

 microscopic examination one is surprised to find very little of the dye stored within 

 the chorionic epithelium. If the animal has not been deeply stained one may fail to 

 find any at all. In a well-stained guinea-pig, it can be distinguished as a fine blue 

 tracery in the chorionic epithelium of the placental labyrinth (fig. 3). The stain is 

 faint, however, as compared with its exhibition in the same cells of the mouse and rat. 



In the giant cells and syncytium of fetal origin which invade the decidua and 

 form an irregular boundary between the fetal and maternal tissues, commonly 

 referred to as the central zone or "Umlagerungszone," no trypan-blue granules are 

 found. This is surprising, since in the injected mouse and rat these cells are filled 

 with blue pigment throughout gestation. Possibly the fetal syncytium stains 

 vitally in the guinea-pig in younger stages, when the ectoplacental cone is sending 

 out syncytial roots and is burrowing its way into the decidua. However, in the 

 youngest fetus (15 mm.) observed in these experiments no dye was found, nor was 

 any found in the syncytium which, in the guinea-pig, divides the placental laby- 

 rinth into numerous lobules, the so-called interlobular syncytium. 



We have already mentioned that the roof of the central excavation, a structure 

 peculiar to the placenta of the guinea-pig, stands out in the gross as an unstained 

 area. When the mesoderm containing allantoic vessels invades the ectoplacenta, 

 the latter is virtually hollowed out and a central core of mesodermal tissue is formed. 

 This has been termed by Duval the central excavation. Part of this mesoderm 

 finally invades the central zone, hollows out the masses of trophoblast situated there, 

 spreads out over a broad area, and sends into the decidua a number of villous 

 processes covered by several layers of ectodermal cells. This conspicuous structure, 

 the function of which is unknown, has been designated by Duval as the roof of the 

 central excavation. It persists throughout gestation but the decidual tissue which it 

 embraces between its slender, finger-like processes gradually degenerates and, as 

 term approaches, becomes converted into a homogeneous substance. The villi 



