The Fusion of the Cardiac Anlages in the Duck, Anas.

<P>Blood-cells: extraembryonically it is found in very early stages, but it first appears in the heart, in the 13 - 14 somite stage. Pericardial cavity: the first sign of its appearance is indicated in the 2 somite stage. It commences simultaneously in the multiple foci, and thus formed intramesodermal spaces become confluent to form a single pericardial cavity. Angioblast: it is derived from the cardiogenic plate, and first appears in an embryo with 3 somites. It is fundamentally bilateral in origin, and at very early stage are the two sides anastomosed with each other in the cranial region. Myocardium: it is formed from paired cardiogenic folds. There can not be noted any distinct demarcation on the surface of the myocardial tubes before they are fused together. The folds are first fused at the middle part in the 7 - 8 somite stage, and the fusion is completed in the 12 somite stage. Mesocardium: the ventral mesocardium first appears in the embryo having 6 - 7 somites, and ruptures at its middle part in the 8 - 9 somite stage. Its remnant is observed in still later stages, namely, we can find it in an embryo with 12 somites. The dorsal mesocardium formes between the 9 and 10 somite stages, and rupture between the 15 and 18 somite stages. Endocardium: it is formed from angioblasts. It has the endothelial character in the 5 - 6 somite stages, and the paired tubes are derived from it in the 9 somite stage. The endothelial tubes unite in the bulbar and the ventricular region. From the network of angioblasts which are spread at the cranial portion of the embryonic shield from which a pair of aortic arches are derived. In the bulbar region, the right endothelial tube markedly exceeds in caliber that of the left side. The latter atrophies and disappears, and the former makes the genuine bulbus cordis. Caudally, on the contrary, the left tube exceeds that of the right and the former is situated sinistro-ventrally to the latter. In the 13 - 14 somite stages the fusion of the paired tubes is almost completed, and the ventricle and atrium may be distinguished by the atrioventricular constriction. In closing, I wish to express my cordial thanks to Prof. Dr. J. Shikinami for his kind criticism and advice.</P>