Abstract
Although the relative functions of the yolk-sac and allantoic placentas in rodents are still uncertain, the following considerations indicate that the former has greater importance than is generally recognized. Goldmann 1 observed that the highly colloidal acid-azo dyes do not enter the chorionic villi of the rat placenta, but pass through Reichert's membrane and are stored superficially in the “yolk-sac epithelium.” Brunschwig 2 has demonstrated that the yolk-sac is the route of entrance of iron salts into the 9-day blastocyst. Furthermore, the present experiments show that the neutral red accumulation in the yolk-sac epithelium exceeds that in any other embryonic tissue when an early blastocyst is placed in a solution of the dye. Also, when embryonic loci of from 11 to 13 days'gestation are perfused by way of the uterine artery with a solution of toluidin blue in saline, the dye passes into the embryo by way of the yolk-sac and the .vitelline veins. When pregnant females of 16 or 18 days are injected intravenously with trypan blue, the only embryonic tissue in which dye is found 1 to 2 hours later is the portion of the yolk-sac epithelium which rests against the placental disc. The dye diffuses through the base of the placenta, rather than through the extremely thin “plates” of the chorionic villi.
The yolk-sac epithelium is composed of columnar cells. An ellipsoidal or spherical nucleus is located at the base of each cell until the 10-day stage and then is displaced upward by a cluster of fat droplets. Beneath the cuticular border of the cell is a granule-containing layer. Between it and the nucleus lies a region which is vacuolated in fixed material.
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