110 University of California Publications. [zoology. 



toward which the contractions were travelling; but the result of 

 these experiments was always negative. The cuts did not change 

 the direction of the contractions. Finally, to make the consider- 

 ation of the evidence complete, allthe cuts made were considered 

 from this point of view, and it was found that in the great 

 majority of eases the direction of the contractions did not change 

 after the cut. In a Eew eases it did change, but the character of 

 the change was not a constant one, for after the change the 

 direction of the contraction was toward the most recent cut, 

 exactly as frequently as it was away from it. It is very clear 

 flan, that flu stimulus due to isolating tin- pints cannot account 

 for tin persistence of the fixed direction of contraction. 



Another possible explanation of the phenomenon is that it 

 may be caused by the refractory qualities of the Ciona heart dur- 

 ing and immediately following each contraction. Schultze (1901) 

 found that the Salpa heart has a refractory period, similar to 

 that found in Vertebrates, during which no stimulus could pro- 

 voke an extra contraction. Now if a piece of beating Ciona 

 heart be isolated, one end of it will have finished contracting 

 before the other, and might consequently be in a better position 

 to originate spontaneous contractions than the other. The con- 

 tractions would consequently start at the end of the isolated 

 piece which formerly contracted first. That is, they would con- 

 tinue in their original direction. 



This possibility was tested in two ways: 



In the first place, if this explanation is correct, then the 

 direction of contractions immediately preceding isolation is the 

 important thing, and it should not matter whether the piece was 

 isolated during a normal series, or after the direction of the con- 

 tractions had been rigidly fixed by first removing one end. 

 Accordingly pieces of the heart were isolated during the normal 

 series, and it was found that in 13 hearts out of 21 (or 62 per 

 cent.) the pieces did not change their direction after isolation. 

 These results seem to indicate that there is a tendency for pieces 

 of the heart isolated during the normal series to maintain the 

 original direction of contraction after isolation; but this ten- 

 dency is much weaker than in those cases where the direction 

 was fixed before isolation. 



