REGENERATION 



169 



The inductive power, therefore, 

 is not a property of one definite, 

 specific tissue only, but it is due 

 to a physiological condition the 

 intensity of which decreases as a 

 gradient from the rostral, or 

 apical, towards the caudal, or 

 basal, end of the body. In other 

 words: each part of the field in- 

 fluences the more caudal (basal) 

 parts, but is itself influenced by 

 the more rostral (apical) parts. 



Little is known so far about the 

 physical nature of this gradient, 

 though it is obvious to think of 

 electro-physiological phenomena in 

 this connection. The recent work 

 of Moment (1946-49) is highly im- 

 portant in this respect. We have 

 seen above (p. 162) that a re- 

 generation-bud at a hind edge as 

 a rule completely replaces the lost 

 part of the body. Now Moment has 

 shown that during regeneration of 

 a caudal half in the earthworm 



W^ 



Fig. 61. Organisation by 

 transplanted parts of the 

 stem in the hydroid polyp 

 Corymorpha. (a) an apical 

 part of the stem induced a 

 complete new hydranth in 

 48 hours; ib-c) more basal 

 parts of the stem induced 

 smaller (&) or abnormal 

 outgrowths (c) in the same 

 time. After Child. 



Eisenia foetida the formation of 

 new segments stops when the total number of segments has 

 reached an approximately normal value. In a normal worm, 

 there is an electric potential difference between the rostral and 

 caudal ends of the body. This potential is destroyed by amputa- 

 tion, but during regeneration it is gradually built up again 

 until it has regained its original value, which happens at the 

 very same moment at which the formation of new segments 

 comes to a stop. On this basis. Moment concluded that the 

 production of new segments is brought to a close by an in- 

 hibitory action exerted by the electric field as soon as this has 

 attained a certain degree of intensity. 



Two parts originally situated at different levels in the 



