The development of the truncus and the conus was studied in the chick embryo by in vivo labelling techniques. The earliest labels were placed at the stage of fusion of the myocardial troughs (stage 9-) and they were traced until the mature heart stage (stage 35). Microdissections and light microscopic studies were also carried out. The results are discussed in relation to the human heart. Our experiments permit the following conclusions: (1) At stage 9- fusion of the myocardial troughs takes place at the level of the primordium of the trabeculated portion of the right ventricle, when neither the conus nor the truncus are present. (2) At stage 12 (loop stage) there appears the caudal portion of the conus, which constitutes the cephalic end of the cardiac tube. (3) The truncus appears between stages 13 and 22. (4) At stage 22 angular junction between the conus and the truncus is the area where the semilunar valve cusps of the great arteries will develop and that, at this same stage, the junction between the conus and the trabeculated portion of the right ventricle seen from the right surface corresponds to the inferior edge of the crista supraventricularis. (5) It was confirmed that the pulmonary semilunar valve cusps originate from the walls of the truncus. (6) The development of the conus and truncus are similar in chick and man. (7) Histologically, in the chick, the wall of the truncus and the conus contain cardiac muscle as late as stage 28, but from then on the walls of the truncus are transformed into connective tissue and plain muscle.
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