196 PATTERNS AND PROBLEMS OF DEVELOPMENT 



not direct effects of physiological or external factors inhibiting head de- 

 velopment but represent secondary differential reactions of the develop- 

 ing head region following a primary inhibition. 



There is, at present, no evidence of specificity in the differential modifi- 

 cations of head form. It was noted above that they do not concern any 

 particular organ or region of the head but involve any and all parts. The 

 differential inhibitions of head form constitute a continuous graded series 

 ranging from slight inhibition of the median region to complete inhibition 

 of the whole head. The inhibiting factor, whether physiological or an ex- 

 ternal agent, acts, of course, on the earlier stages of head regeneration be- 

 fore the various organs are present as localized differentiations. The sec- 

 ondary modifications, although appearing under a rather narrowly limited 

 range of conditions, evidently also constitute a continuous graded series. 

 The appearance of the same series of forms with physiological factors and 

 with many external agents suggests that the effects of all these different 

 factors on the primary pattern of the planarian head must be essentially 

 similar. In other words, the only possible conclusion, in view of all the 

 evidence, seems to be that the mediolateral pattern which is differentially 

 modified must consist primarily in a quantitative gradient or differential. 

 If specific differences are present in the cells from which the head develops, 

 they are not concerned in the differential modifications of form and pro- 

 portion. However, the differentiations which appear in the modified heads 

 suggest that the quantitative gradient provides the physiological basis 

 for differentiation, and the differential susceptibility of different levels 

 of this gradient to physiological and external factors determines not only 

 form and proportions of the modified heads but the differentiations which 

 appear in them. 



Absence of physiological differential inhibition of head development 

 ii;i certain triclads may be due to one or more of several factors : activation 

 of the head-forming cells may be so intense that the nervous factor cannot 

 prevent it, or the nervous factor may be slight or less readily transmitted 

 anteriorly in some species than in others; in some species head regenera- 

 tion is more completely isolated from the old nerve cords by regenerating 

 tissue posterior to it; in some the head develops so slowly that the transi- 

 tory nervous effect of posterior section may disappear before it can be 

 effective in inhibiting head development. Only further experiment with 

 the various forms can throw light on these points. 



