150 LIBBIE H. HYMAN 



high during the time of head determination ; and it is these pieces 

 which produce inhibited structures. 



3. General conce/ption of the process of regeneration 



This relation between frequency of normal head formation, 

 and rate of metaboHsm at the critical period of head determi- 

 nation in Lumbriculus has lead me to accept the conclusions to 

 which Child has come from the consideration of similar facts in 

 Planaria dorotocephala. Since he has already presented his 

 conclusions (Child, '14e), it is not necessary for me to discuss 

 them in any very great detail. For the following brief present- 

 ation, it will be convenient to employ a figure similar to that 

 frequently used by Child (fig. 24). The cells at the cut surface x 



Fig. 24 Diagram for theory of head formation. 



as a result of the wound and altered conditions begin to produce 

 new tissue from which the new head is to arise. These cells 

 grow out with a certain rate of metabolism w^hich is relatively 

 high as shown by disintegration experiments. Now if the rate 

 of metabolism of the old tissue tj is low, there is nothing to hinder 

 .r from continuing its development; it becomes dominant over y, 

 uses up the material of y for regeneration, and produces a normal 

 head. This is the case in anterior pieces of Lumbriculus where, 

 as was shown in the preceding section, the rate of y is low during 

 the time that the head is determined as normal. If, on the con- 

 trary, the rate of y is high, then x will not be able to dominate over 

 y, nor to attain sufficient independence to produce a normal head; 

 the development of x will be inhibited, and in proportion to the 

 metabolism of y. Thus are produced the various types of inhib- 



