An electronmicroscopic study of the truncus ridges in chick embryos.

In 6 days' chick embryos, an electronmicroscopical study was made of the fusion of the endocardial ridges of the truncus arteriosus. It could be demonstrated that in this fusion process not only the endothelial cells are involved, but also cells of the condensed mesenchyme of the parietal ridge. The endothelial cells form protrusions which interdigitate and show endocytosis whereas the cells of the condensed mesenchyme produce long, blunt pseudopodia containing filaments and microtubules.