A causal analysis of the heart curvatures in the chicken embryo.

By means of grafts of the heart tube of the chicken embryo (before incurvation), we tried to clear up those factors which determine the curvatures. The heart tube, cultivated or grafted, possesses in itself the property of incurvation but it is manifested only if it is developed in a medium or a cavity similar to the pericardiac cavity. This property cannot manifest itself in the thickening of the tissues such as the embryonic mesenchyme. The curvatures which are formed, that is to say the morphogenesis (in the coelomic grafts) is always imperfect and cannot be compared to those of a normal heart in the same stage of development. The ventricle and the heart bulb can be recognized many times, but not the atrium or the venous sinus. The retrocardiac tissue and the endoderm of the anterior intestinal portal are factors necessary for a normal morphogenesis, as the grafts accompanied by these tissues give rise to better morphogenesis and one can recognize all the fundamental portions of the heart. In these cases a liver is almost always differentiated, originating with the graft. The blood flow has no influence for some time in the heart morphogenesis but it does in the structure of the heart. Only in those cases in which circulation exists are the endocardium and the myoepicardium formed, with their typical structure. The morphogenesis and the heart structure are, to a certain point, independent processes during these stages of development and are due to different factors.