TRANSMISSION OF MODIFICATIONS 399 



This shows, not only that a center of coordination exists in 

 the organ itself, independent of nerve centers, but that the 

 action of one part becomes the impulse for exciting action in its 

 neighboring part. 



Hydromedusae of different rates of pulsation were united in 

 pairs in Loeb's laboratory by the process of grafting. When the 

 union was complete along nearly all the cut edge the whole 

 beat synchronously, but when the union covered but a small 

 area the two beat separately with a different rhythm. 1 



The heart of the ascidian is an elongated tube, beating so as 

 to send the blood alternately from left to right and from right 

 to left ; that is, the impulse seems to originate at one end for a 

 time, and then, after several hundred beats, to shift to the other 

 end. It was found that the " area of impulse " was confined to a 

 short section at either end ; that each of these sections, if cut 

 away, continued to beat rhythmically, but that the longer middle 

 part seemed incapable of contraction without external stimulus. 

 Commenting on the fact, Loeb says : 2 



These experiments, it seems to me, leave no room for doubt that the 

 change in the direction in the contraction of the ascidian's heart is deter- 

 mined by each of the two ends getting the upper hand alternately and forcing 

 the other center to act in its rhythm for a time. This " getting the upper 

 hand " might possibly mean nothing more than that one end gains the time 

 in which to send off a wave of contraction before the other end begins to 

 contract. For this it is only necessary that a single heart beat of the lead- 

 ing end be delayed or fail entirely, a phenomenon that also appears occasion- 

 ally in the human heart. 



Locomotion in the earthworm is by a series of elongations and 

 contractions of the successive segments of the body, in regular 

 order from the front backward. Nobody ever supposed this serial 

 order to be controlled by conscious intelligence, but it has been 

 assumed to be due to the control of nerve fibers from the ganglia 

 along the dorsal surface of the body. If, however, the worm be 

 cut in two and the parts united by threads, locomotion is entirely 

 successful, even though a considerable space intervenes between 

 the pieces. In this case contraction proceeds backward, seg- 

 ment by segment, as in uninjured worms, suffering no special 



1 Loeb, Physiology of the Brain, pp. 26-27. 2 Ibid. pp. 27-29. 



