EXPERIMENTAL STUDIES ON FETAL ABSORPTION. 51 



to find any stain at all in the adrenals, testes, etc. We are aware that the staining 

 in these fetuses does not represent the extent to which it would occur if repeated 

 introduction of the dye were made. Such a procedure is technically impossible. 



The appearance of the vitally stained fetus in sagittal section is of interest 

 (fig. 4). It is seen at a glance that the central nervous system remains unstained. 

 This behavior is identical with that in postnatal life. The physical conditions which 

 prevent the passage of foreign colloidal material from the blood-stream into the 

 central nervous system appear to be established relatively early in intrauterine life. 

 The cells within the fetus which have been observed to store trypan-blue granules 

 may be conveniently described under three headings: (1) endothelial cells, (2) con- 

 nective-tissue cells, and (3) epithelial cells. The storage of the dye in the fetal 

 membranes and placenta will be described separately. 



Endothelial cells. — The only endothelial cells of a fetus 30 to 40 mm. in length 

 which have been noted to absorb trypan-blue under the conditions described, are 

 those lining the sinusoid spaces of the liver. Characteristic blue granules are always 

 abundantly present within the cytoplasm of these cells. 



Connective-tissue cells. — Large, round cells, with a single round or oval nucleus, 

 whose cytoplasm contains granules of vital dye, are occasionally encountered in the 

 connective tissue. It is noted that, as in the adult, the elastic tissue of the fetal 

 arteries exhibits an affinity for the dyes. 



Epithelial cells. — The kidneys of a fetus 30 to 40 mm. in length are striking 

 because of the large quantity of dye invariably found within the epithelial cells of 

 the convoluted tubules (fig. 7). The dye is most abundant in the proximal portion 

 of the tubules, scant in the distal portion, and entirely absent in the collecting 

 tubules. The glomerular epithelium remains unstained. The fetal urine gives 

 the characteristic stain of trypan-blue on filter paper. 



It may be said, therefore, that the fetal liver and kidneys play an important 

 part in the removal of foreign colloidal material from the fetal circulation. 



Trypan-blue appears in the form of granules within cells in the umbilical cord, 

 amnion, yolk-sac, and placenta. 



Umbilical cord. — In the umbilical cord the dye appears in the form of fine 

 granules in the mucoid connective tissue (fig. 8). These granules are aggregated 

 in the cytoplasm of the cell on both sides of the nucleus. 



Amnion. — Trypan-blue occurs abundantly in the cytoplasm of the amniotic 

 epithelium in the form of small blue granules (fig. 9). Bondi (1905) observed similar 

 granules in the human amnion after staining it with neutral red. He believed that 

 this appearance signified a secretory activity on the part of the amniotic epithelium. 

 In the light of our present knowledge, the fact that the amniotic membrane stains 

 with neutral red and trypan-blue indicates that its cells possess cytoplasmic vacuoles 

 capable of absorbing and storing these substances. As to the normal role of these 

 cytoplasmic vacuoles very little is known. The mesothelial cells attached to the 

 inner surface of the amniotic epithelium are also laden with fine blue pigment (fig. 9). 



Yolk-sac. — In the guinea-pig the inverted yolk-sac or vitelline membrane be- 

 comes the outermost fetal membrane and a layer of endothelial cells consequently 



