Normal and Thorotrast‐injected chick yolk‐sac specimens were prepared for electron microscopy and examined at intervals from early in the incubation period through the twelfth day after hatching. At all stages of development studied, it appeared that electron‐dense yolk granules, lipid droplets, and Thorotrast particles were phagocytosed by the entodermal lining cells through a complicated system of coated vesicles and apical canaliculi which were associated with larger vesicles situated more deeply in the cells. The numerous vesicles in these cells frequently appeared to have merged with one another, and absorbed yolk materials within many of them appeared to have become structurally altered. In the late‐incubation and post‐hatching periods, numerous lightly osmicated lipid spherules (50–150 mμ diameter) were seen within cisternae of rough‐surfaced endoplasmic reticulum, in vesicles of the Golgi apparatus, and in membranous enclosures near the basal plasmalemma of the entodermal cells. These lipid spherules were also present within intercellular spaces and within endothelial cells of vitelline capillaries, suggesting a sequence of events in which spherules were released from entodermal cells into the extracellular space and then taken up by the vascular system for further transport.
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Roger O. Lambson (1970) studied this question.