The question whether the pace-making and conduction systems reside in the remains of primitive portions of the cardiac tube in an undifferentiated form, or whether on the other hand these primitive portions develop into differentiated structures which preside over these functions may be reviewed afresh. Obviously the tube in its early state possesses these functions; obviously also the major part of the heart loses them during the course of development. A knowledge of the changes in form paralleling changes in function would have great interest. On this phase of the problem we hope to report later.

On the basis of these observations, differentiation from the point of view of stimulus production may be viewed perhaps in this manner. Pace making, the conduction of impulses, and contraction are the primitive functions of the tube. As the tube develops into the adult structure, pace making and conduction are supposedly served by tissues resembling in structure the original ones. Whether as a matter of fact a structural change takes place is an interesting and important problem. Those portions of the heart which require to develop greater degrees of energy lose the primitive functions of pace making and conduction, and, in the transformation, take on a differentiated structure. It is, then, not the structures in which the primitive functions of pace making and conduction reside which are differentiated, but the greater mass of ventricular muscle. These reflections have their origin not only from our own work but they grow out of observations to be found in the writings of those (A. Keith and I. Mc-Kenzie) who call the nodal and conduction tissues in the heart, embryonic. But whether from the point of view developed here the use of this term is completely descriptive remains an interesting problem.