DOMINANCE AND PHYSIOLOGICAL ISOLATION 329 



gradient. In early development, however, contraction begins in the ven- 

 tricular region and appears progressively toward the sinus and finally at 

 the sinus. This probably does not represent a progressive shift of domi- 

 nance from the ventricular to the sinus region but results from the course 

 of growth and differentiation of the myocardium (see, e.g., Copenhaver, 

 1939). Dominance of the sinus does not become evident until contractility 

 has developed over the whole heart. If the sinus of the fully functional 

 heart is inhibited, physiological isolation results, and a beat is initiated 

 at a lower level. As in the ctenophore plate row, physiological isolation 

 of subordinate regions with resulting independent contraction can also 

 be brought about by blocking the impulse from the sinus by direct stimu- 

 lation of the part concerned. By transplantation of the dominant sino- 

 atrial region to the conus region in the embryonic chick heart in vitro, 

 reversal of direction of beat and determination of ventricular rhythm by 

 the transplanted sino-atrium have been brought about (Paff, 1936). 



In animals with tubular heart and periodic, or occasional, reversal in 

 direction of passage of contraction along the heart — for example, ascidians 

 and various arthropods — the pacemaker or dominant region is not fixed 

 in position (Gerould, 1931, 1933; Wolf, 1932). The end which is domi- 

 nant at a certain time loses its dominance sooner or later, in consequence 

 either of fatigue or of some other change in condition; and the other end, 

 becoming physiologically isolated, becomes independent, initiates a 

 rhythm, and becomes dominant and the pacemaker before the region 

 originally dominant regains its original condition. Later the same decrease 

 in dominance occurs in the new pacemaker, and reversal in direction of 

 beat again results. As already noted, a similar reversal in direction of 

 transmission in the plate row of the ctenophore Pleiirobrachia can be in- 

 duced by differential inhibition and recovery or by direct stimulation of 

 subordinate regions. In differentially inhibited fish embryos the heart 

 may remain tubular with reversible beat (Gowanlock, 1923). In all these 

 cases with reversal of beat the heart gradient is apparently readily re- 

 versible, either periodically under natural conditions or experimentally 

 by physiological isolation of a previously subordinate part in consequence 

 of differential inhibition or of direct stimulation of the part. 



At certain definite levels of the mammalian alimentary tract are regions 

 which, under ordinary conditions, dominate levels posterior to them for 

 a certain distance and may be more or less independent of each other or, 

 under other conditions, may be subordinated to the dominance of more an- 

 terior regions or to the most anterior dominant region. Under pathological 



