Observations on the source of embryonic myocardioblasts.

The light and electron microscopic appearance of mesenchymal and myoblastic tissue in the embryonic heart is described. The similarity in the morphological appearance of all mesenchymal cells is pointed out, whether they occur in atrioventricular cushion tissue, the ridge tissue of the bulbus, cordis, sub-epicardial tissue or in mesenchyme elsewhere in the embryo. Cells intermediate in their ultrastructure between mesenchymal cells and myoblasts are found. The significance of such cells is discussed and the suggestion made that their appearance is consistent with their representing stages in the differentiation of mesenchymal cells into myoblasts.