Studies en Regulation. 537 



tissue develops according to the manner in which it is used by 

 the animal or piece. In the presence of the cephalic ganglia or a 

 sufficiently large portion of the ganglionic tissue the anterior end 

 of the piece is used in a characteristic manner and this functioning 

 must subject the new tissue to a characteristic complex of con- 

 ditions. It is not my purpose to attempt a description or enu- 

 meration of all the possible factors concerned in the differentiation 

 of the parts; such an attempt would be at present in large part a 

 series of surmises. Among these factors, however, are functional 

 nerve stimuli, chemical and physical conditions in the tissues 

 resulting from them, all the complex of metabolic factors as in- 

 fluenced by the particular conditions, and the conditions, mechan- 

 ical and otherwise, connected with and resulting from motor 

 activity or attempts at such activity. 



The relative importance of different factors differs widely in 

 different cases. In the present case the motor activity determined 

 by the presence or absence of the ganglia is of great importance. 

 It is only on this basis that we can explain the fact that anterior 

 to the ganglia the amount and rapidity of regeneration are directly 

 proportional to the amount of material removed. We have, I think, 

 no adequate conception of the complex interactions leading to the 

 formation of such a structure as the head of Leptoplana, but that 

 motor activity is an important factor, mechanically and perhaps 

 otherwise, cannot be denied. The constantly varying physical 

 conditions resulting from motor activity may seem incapable of 

 giving rise to any definite or characteristic form, but the important 

 fact is that, notwithstanding their constant changes, they con- 

 stitute a characteristic series frequently repeated. From the time 

 when the first traces of motor activity appear in the new tissue 

 up to complete development the animal is using or attempting to 

 use this part in a definite characteristic manner. The arrange- 

 ment of muscles and the nerve connections are of course directly 

 responsible for this characteristic motor activity, but the charac- 

 teristic arrangement of these structures can be accounted for only 

 by other characteristic functional activities and their correlations, 

 and so on. We are led finally to a protoplasm possessing certain 

 elementary and characteristic activities, but that any substances 



