The early development of the heart of Tupaia belangeri, with reference to other mammals.

The development of the heart of Tupaia belangeri from the first endothelial-lined lumina to the cardiac loop is described in 20 embryos with 2 to 14 somites, from ontogenetic days 11 and 12. Bilateral endocardial tubes transporting blood are found in the 8-somite embryo; in the middle cardiac plate, angioblasts and angiocysts are located between them. In the 9-somite embryo, formation of the cardiac loop has started, the endocardial tubes approach each other closely, most of the angiocysts have been incorporated by the expanding endocardial tubes, and fusion of the endocardial lumina has started in the cono-truncal area. Apparently, much of the endocardial cardiac loop found in the 9-somite embryo has been produced by the disproportionate lengthening of a segment of the endocardial tubes, which is very short in the 8-somite embryo. In the 13-somite embryo the endocardial tubes have largely fused, but tube-like strands of endothelia, remnants of the original endothelial walls separating them, form a "palisade" and mark the original boundary between them. Myoepicardial differentiations of the splanchnopleure begin separately on both sides of the embryo and gradually spread craniad until they coalesce in the midline, in front of the anterior intestinal portal. The caudal portions of the endocardial tubes with initial myoepicardial and cardiac jelly differentiations do not contribute to the definitive heart. The anterior intestinal portal is very broad in Tupaia. Contradictions in the literature as to the bilaterality of cardiac primordia of eutherian mammals are discussed. The hypothesis is developed that bilateral endocardial tubes and bilateral myoepicardial differentiations of the splanchnopleure develop in species with a large yolk-sac, relatively late closure of the foregut, and a broad anterior intestinal portal (e.g., Tupaia, ferret, and cat, etc.). This is probably the primitive condition in eutherian mammals. In species with a small yolk-sac and/or reversal of germ layers (man, rodents), the foregut and anterior intestinal portal are formed earlier, the heart primordium reaches its median position ventral to the foregut in the angiocyst-stage, and the first endocardial lumina appear close to the midline. In these species, the primordium of the endocardium seems to be plexiform and without clear evidence for bilaterality.