Valvular action in the embryonic chick heart by localized apposition of endocardial masses.

The heart of a chick embryo at the time the blood first starts to circulate through it is essentially a double-layered tube. I t s endocardia1 layer consists of an endothelial lining held in place within the outer epimyocardial investment by a surprisingly thick layer of gelatinous, non-cellular material which Davis (’24, ’27) called cardiac jelly. Blood enters this primitive young heart at its caudal end and is forced out its cephalic end by peristaltoid contractions of the myocardium. The fact that the cardiac tube soon becomes bent on itself does not at first essentially change the way the blood passes through its unchambered length. Nor does this early tubular heart have anything resembling valves as we know them in the adult heart. It is not surprising, therefore, that when the recently established circulation is studied in living embryos one sees a very jerky progression of the corpuscles in the vessels entering and leaving the heart. There is with each cardiac cycle what one might call a “backlash.” This term in this connection is meant to convey the idea that toward the end of each contraction cycle the corpuscles in an artery lose a little of the progression they have made under the thrust of systole. I n veins a similar blacklash occurs as contraction commences at the atrial end of the cardiac tube.