What are the factors which lead to the fusion of the two heart primordia ? An experimental analysis.

1. In stages 5 and 6 ofHAMBURGER andHAMILTON, the presumptive heart primordia of both sides, placed in contact, Fig. 1, originate two hearts in many cases instead of joining and originating a unique heart, as happens during normogenesis. 2. The two hearts are independent and are found to be very close in the same pericardial cavity, Figs. 7 and 8. 3. The heart formed, from the primordium left "in situ", is a complete and normal heart, Figs. 7 and 8. 4. The heart formed from the transplant is much smaller and consists of a bulb, ventricle, and in some cases atrium, but its curvatures are typical and normal, Figs. 7 and 8. 5. The heart formed of the transplant organizes itself in contact with a small foregut formed from the endoderm of the transplant, Figs. 4 and 9. 6. The endoderm ist not only important in order for the P.H.M. to begin its differentiation in a heart direction but also organizes the P.H.M in such a way that the result is not only heart tissue but a heart, i.e. an organ with typical form and structure.