DIFFERENTIAL DEVELOPMENTAL MODIFICATION. I 193 



lateral differential inhibitions of development. The facts suggest a grada- 

 tion decreasing from the median region laterally as regards the "physio- 

 logical level" necessary for initiation of development of the head. That 

 this level involves metabolism seems evident. The action of both physio- 

 logical and external factors in inhibiting head development apparently 

 consists in preventing activation of the cells concerned to the level re- 

 quired. The inhibition of development involves progressively regions far- 

 ther lateral as the action of the inhibiting factor increases. 



This interpretation will serve for the longer pieces on which no in- 

 hibited heads appear under natural conditions. As regards the shorter 

 pieces, however, it appears, at first glance, to be in contradiction to the 

 results of experiments, for agents which inhibit head development differ- 

 entially in longer pieces with high head frequency usually have the op- 

 posite efifect on shorter pieces with a naturally low frequency. These 

 agents decrease the mediolateral differential inhibition even though they 

 retard development of the head. These effects of external agents indicate, 

 and experiments on delay of posterior and anterior section, to be pre- 

 sented later (pp. 406-11), provide conclusive evidence, that in the shorter 

 planarian pieces two antagonistic factors are concerned in determining 

 the head form on a particular piece : the one the activation of the cells 

 concerned in head formation; the other the effect, nervous stimulation, 

 or whatever it may be, resulting from section of the nerve cords at the 

 posterior end of the piece. This effect is probably not essentially different 

 from a functional stimulus which tends to maintain the cells as cells of a 

 particular body-level. In order to give rise to a head, they must become 

 free of relations to other parts. Removal of more anterior regions has 

 freed them from relations to those parts; but if nervous stimuli from more 

 posterior regions are sufficient to keep them, to some extent, functional 

 parts of a particular body-level, they do not attain the level of activation 

 necessary for development of a normal head, and the inhibition series 

 of head forms results, according to the effectiveness of the physiological 

 factor. Transverse section of the longitudinal nerve cords with minimum 

 injury of other parts, if within a certain distance of the anterior end of 

 the piece, decreases head frequency; but transverse section of other re- 

 gions at the same level has little or no effect on frequency (Watanabe, 

 19356). Also, removal of a short piece half the body width, involving 

 section of one nerve cord a short distance posterior to a level of head 

 regeneration, usually results in asymmetry of the head, the side anterior 

 to the half-section being more or less inhibited (Rulon, 19366). 



