Structure of the intra-chorionic blood sinus in the chick embryo.

Portions of the chorio-allantoic membranes from 15 day old chick embryos were processed for electron microscopical examination. The analysis of both 1 micrometer thick sections stained with toluidine blue, and of thin sections stained with uranyl acetate and lead citrate, showed that the lumen of the intraepithelial vascular spaces in the chorion constitutes a single cavity extending over the whole membrane. The vascular arrangement can thus best be described as a single blood sinus, and not as a network of capillaries or sinusoids. The large lumen of the sinus is interrupted by cylindrical columns connecting its floor with its roof. Each column is enveloped in a layer of endothelium, a basal lamina intervening. The core of the column is formed by cytoplasm from two to five different cells ('villuscavity' cells, 'capillary-covering' cells or various combinations of both).