The hearts of chick embryos from stages 9 to 11 according to Hamburger and Hamilton are studied by means of the electron microscopy, and the arrangement of the cardiac jelly (CJ) during the different steps of the fusion of the paired heart anlage is described. It is shown that two different areas can be distinguished in CJ: the CJ located between the endocardium and the myocardium (MECJ) and the CJ located between the endocardium and the ventral foregut endoderm (EECJ). MECJ is a zone poor in ultrastructural components most of which are unhanded filamentous material and low-density amorphous material, and its arrangement remains constant during the whole fusion process; EECJ, on the contrary, is very rich in ultrastructural components containing greater amounts of high-density amorphous material, collagen fibrils and cellular detritus, and the arrangement of this area of the CJ undergoes changes during the different fusion steps. Alcian blue staining does not show differences between both areas of CJ. It is suggested that the CJ can play a role during the fusion of the heart. In addition, some observations are reported which suggest that all the epithelial tissues surrounding the CJ can take part in the elaboration of that extracellular material.