Small fragments of mesoderm and endoderm have been transplanted from stage 5 donor embryos labeled with thymidine-³H to corresponding sites in nonlabeled hosts. These fragments heal in and participate in the normal development of the recipient embryo. The distribution of the implanted cells in the tubular heart at stage 12 has been determined by autoradiography, to provide a set of corresponding points in the stage 5 precardiac mesoderm and the epimyocardial and endocardial layers in the stage 12 heart. The tubular heart has been subdivided into 24 defined regions, and the boundaries of those regions have been mapped in the stage 5 mesoderm. The process of heart formation has also been traced through intervening stages by graphic reconstruction and microdissection of the mesoderm layer and by marking the epimyocardial troughs and forming heart tube with particles of iron oxide.

The preepimyocardial mesoderm is organized in the stage 5 embryo in two separate regions, one on either side of the embryonic axis, with a gap of noncardiogenic mesoderm of about 0.8 mm between them. At this stage preendocardial cells are organized into areas generally similar in size and location to the preepimyocardial regions, but they are more widely scattered.

Each lateral heart-forming region can be subdivided into a series of curved bands. The rostromedial band of each heart-forming region forms the truncoconal tissue of the tubular heart. The mid-portion gives rise to the ventricles and the caudolateral area of each region contributes the cells which form the atria.

During cardiogenesis, the preepimyocardial mesoderm behaves as a coherent sheet. It condenses, stretches, folds, and deforms, but it does not lose its integrity as a sheet, nor its continuity with the rest of the layer of splanchnic mesoderm. It does not break up into cells or cell clusters. Endocardial cells, on the other hand, do show some evidence of dispersal as singlets and small groups during formation of the heart tube.