THEORIES OF REGENERATION 



235 



Fig. 159. Two theories of re- 

 generation as applied to hydroids. 

 Top: A "T" substance is assumed 

 to be present in the upper part of 

 the hydroid and a "B" substance in 

 the lower part. The "T" substance 

 causes tentacles to form; the rr B" 

 substance causes base to develop. 

 When a cut is made through the 

 middle of the hydroid, the cells on 

 either side of the cut must, accord- 

 ing to the above assumption, have 

 the same amount of "T" and "B" 

 substances. However, the cells on 

 one side develop into base, while 

 the cells on the other side develop 

 into tentacles. Bottom: A substance 

 is assumed to be present through- 

 out the hydroid but varies in con- 

 centration — 5"„ near tentacles, S nA 

 in middle, and S n _ 2 near base. After 

 the hydroid is cut into two parts the 

 tentacles develop at the end where 

 S is present in highest concentra- 

 tion. It is not the absolute concen- 

 tration of S which is important but 

 rather the relative value in any one 

 piece of the hydroid. 



W 



Sn 



-S n - 



Sn-i 



Sn-. 

 Sn-i 



2 



kw 



TENTACLCS 



Sn 



GREATER 

 THAN 



Sn-I 

 <- SYBASE 



\MrTENTACLES 

 Sn-| 



Sn- 



GREATER 

 THAN 



Sn-2 

 J-BASE 



ganization in the egg. Again we consider two possibilities. We may assume 

 that there are different chemical substances responsible for the formation of 

 the tentacles and the formation of the base. Suppose we call these substances 

 T and B, respectively. The two substances then would have to be in the 

 original body from which we isolate parts (Fig. 159); substance T would 

 have to be at the top and substance B at the lower end. At this point we run 

 into a difficulty in our attempt to apply the concept of different substances to 

 regeneration phenomena. Because at the middle region of the body, where 

 we make the cut, B would have to be present on one side of the cut and T on 

 the other side in order to get the tentacles at the top of one piece (2) and a 

 base at the bottom of the other piece ( 1 ) . Now this situation is obviously not 

 possible, because the cells, which on the basis of the theory should have con- 



