126 CM. Child. 



long under these conditions as when both are present. I think 

 there is little doubt that the difference is connected with the func- 

 tional activity of the parts. Each region develops its character- 

 istic form only as it is used in the cjiaracteristic manner. In all 

 cases marked growth anteriorly and laterally of the new margin 

 of the head has been observed as soon as the animal begins to use 

 it in the manner characteristic of these regions, while the case 

 represented in Figure 23 shows that so long as this region is not 

 used in the ordinary manner it may develop posteriorly. The 

 posterior region of the body can perform its functions to some 

 extent in the absence of the cephalic ganglia as will be shown else- 

 where, and moreover, in the cases under consideration the regen- 

 erating posterior region is undoubtedly innervated from the part 

 of the nervous system present. It therefore performs its usual 

 functions, though perhaps less perfectly, before the other ganglion 

 regenerates, and its development proceeds in the typical manner. 

 To put It briefly, the margin of the head develops a characteristic 

 form because used in a characteristic manner, and the body de- 

 velops a different form because it is used differently. The preced- 

 ing experiments are sufficient to show that mechanical tension Is 

 an important factor In morphogenesis in these animals. No one 

 would admit more readily than myself, however, that many other 

 factors may be concerned here and that in other cases the factors 

 may be wholly different. 



Attention has been called In several cases to the apparently 

 greater functional activity of the regenerated parts as compared 

 with the old In later stages. This difference indicates, I believe, 

 a real physiological difference. The old portion decreases in size 

 In consequence of loss of material while the new parts increase 

 In absolute size in the earlier stages and in relative size in later 

 stages. It is not at all Improbable that the functional condition 

 of the reduced old part differs widely from that of the new part. 

 The former, reduced to a fraction of Its former size, Is certainly 

 less plastic mechanically and probably less sensitive to stimuli. 

 The new part Is to be regarded as possessing the qualities of a 

 young and growing organism, the old on the other hand as ap- 

 proaching exhaustion. The difference observed in functional ac- 



