The morphologic expression of each cardiac primordium in the chick embryo.

The cardia bifida in amphibians has the bulboventricular loop of the two hemi-hearts convex towards the mid-line. In birds the two loops have been reported to be convex towards the outside because all the experimental methods of producing cardia bifida in birds, unlike amphibians, impede the closure of the foregut. The morphologic expression of each cardiac primordium was studied without altering en dodermal tubulation or altering it only to the least possible degree. It has been demonstrated that the cardia bifida in birds has the two bulboventricular loops convex towards the mid-line and that the heart loops formed by only one primordium also tend to be convex towards the mid-line. Previous studies have been confirmed in the sense that the caudal portion of the left hemi-heart is the more prominent than the right one and that the left hemi-heart has faster beating rate than the right one until stage 12. When the caudal portion of the left cardiac primordium was removed at stage 9 of Hamburger & Hamilton, in all cases a loop convex to the right was formed. This fact excludes that this portion of the left primordium is the origin of the right bending of the cardiac tube.