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SummaryStructural changes taking place in the chorioallantoic membrane (CAM) of embryonated hen's eggs during incubation were followed up by histological, histochemical and electron microscopic methods. The allantoic sac surrounds the amnionic sac and also the yolk sac by the 7th to 11th day, respectively, and forms together with the chorion the CAM. The mesenchymal layer placing between the chorion of ectodermal and the allantoic layer of entodermal origin develops from the somatic and splanchnic‐pleure of the parietal mesoderm.The CAM is composed of three different layers, i.e. chorionic epithelium, mesenchyme and allantoic epithelium. The chorion comprises 2 layers of cubical epithelial cells. In between the 2 layers capillaries and directly under the shell membrane vascular sinuses can be found. Blood circulating in the sinuses is separated from the air in the pores of the shell membrane by the adacent epithelial cell‐projections of 0,2 μm thickness, the basal membrane (0.1 μm) and the sinus endothelial layer of 0.2 μm thickness. The mesenchymal cells are of star‐like form. The allantioc epithelium is built of one layer of fusiform cells with oval‐shaped or rod‐like nuclei.

The quantitative changes of the yolk, albumen and amniotic-allantoic fluids have been estab-The development of the extraembryonic membranes and ultrastructural changes of the chorio-lished.allantoic membrane (CAM) were investigated in goose during the incubation.