THE FUNCTIOKAL SIONlFICANCE O F THE CARDIAC JELLY IN THE TUBTJLAR HEART O F THE CHICK EMBRYO

When, at about the 40th hour of incubation, the heart of the embryonic chick begins to pump blood it is essentially a simple tubular structure, looped to the right. I t s wall consists of a tube of endothelium within a sleeve of epimyocardium. In living as well as in fixed preparations there is a space between these tmo layers. I n the living condition this space appears optically homogeneous. In fixed preparations it is found to be filled with a delicate non-cellular meshwork of fibrils so fine as to be scarcely.visible under the oil immersion lens. Davis ( '24) named this material cardiac jelly and demonstrated its gelatinous character in vivo. I t s composition and structure in the living condition is still problematical. The present report is not primaily concerned with the composition of the cardiac jelly, but rather with its role in the functioning of the primitive tubular heart. I n the heart of a chick embryo of 40 hours incubation, the cardiac jelly surrounds the endothelium of that part of the heart which is destined to form the ventricle and truncus of later stages (Patten, '22). I n cross section this sheath of cardiac jelly is not of equal thickness. It is distinctly thinner at the primary ventral and dorsal midlines. Cephalically it extends to the aortic roots, and caudally to the atrial region of the heart. In the fixed heart of a chick of this age, there