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Grafting regions of the blastodisc of the chick, labeled with H-thymidine at Stages 5 and 6 ofHAMILTON andHAMBURGER, is a very useful technique for following the morphogenetic movements of the grafted material. Areas (E-M) of the blastodisc of 2.75 by 0.55 mm, grafted in the homologous region of an embryo, participate in its morphogenetic movements. The labeled reversed cardiac area can sometimes be incorporated into the host, and a tubular heart can develop which includes some of the graft tissue, Figs. 3, 4, 5, and 6. The morphogenetic movements shaping the anterior intesinal portal and early foregut can occur despite the reversal of a large rectangle of the endomesoderm, Fig. 6. The coelomic epithelium lining the operated side, a part of the myoepicardium of this side, as well as the endoderm of the ventral portion of the foregut, and, more caudally, half of the anterior intestinal portal are derived from the graft as shown by strong radioactive labeling of the cells, Figs. 3, 4 and 6. The morphogenetic movements of the graft endoderm and mesoderm are independent, the pre-heart mesoderm moving in a cranial direction, while the endoderm extends caudally, Figs. 3 and 4.This study raises an interesting question: Can the original cephalic preconal cardiogenic mesoderm regulate to form sinoatrial tissue (and vice versa) ? The evidence obtained in the present investigation is not extensive enough to warrant conclusions as to the important question of "regulation"; for this a considerable number of operated embryos is required in which a recognizable normal heart has developed (up to Stage 12-13).

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