io2 C. M. Child. 



It is possible that the effect of the functional conditions may 

 be in many cases largely mechanical, i. e., that in consequence 

 of the use or attempt at use of a growing part, e. g., a 

 regenerating tail, it is subjected to certain mechanical condi- 

 tions of tension and pressure and that these mechanical condi- 

 tions themselves constitute in reality the chief "formative factor," 

 acting either mechanically or as physiological stimuli to growth. 

 In many cases, however, there is no doubt that other internal 

 stimuli bring about growth, but even in such cases mechanical 

 conditions must usually play a certain part in the final arrange- 

 ment of the material produced. In short there must usually and 

 perhaps always be a mechanical factor of more or less import- 

 ance in regulative morphogenesis. I think it probable that in 

 the lower animals this mechanical factor is relatively simple but 

 of great importance, while with increasing complexity it becomes 

 more complex and more difficult of analysis, though perhaps not 

 less important. 



The alteration in general outline and proportion of pieces, 

 especially of the old portions, called by Morgan morphallaxis, 

 which occurs during regulation in such forms as Planaria (Mor- 

 gan, 'oo, 'oi), Stenostoma (Child, '02, '03) and Leptoplana, I 

 believe to be primarily due to mechanical factors connected with 

 locomotion and acting very probably both in a simple mechanical 

 manner and as stimuli to growth, though there is some reason 

 to believe (Child, '02) that the direct mechanical effect is pre- 

 dominant in many cases. We cannot conclude, however, that all 

 phenomena which have been designated as morphallaxis are due 

 to similar conditions. The changes in form of pieces of the 

 medusa Gonionemus for instance (Morgan, '99) cannot be due 

 to the factors which cause the change of form in Stenostoma and 

 Planaria, but are very probably due to physical conditions in the 

 tissues whose equilibrium is destroyed by a removal of a part, 

 and so may be comparable to the inrolling which occurs in pieces 

 of Cerianthus (Child, '04a). In dealing with problems of so 

 great complexity generalizations are safe only so far as the actual 

 facts go. Nothing is gained by referring these diverse phenom- 

 ena to an inherent capacity in pieces for returning to the original 

 form. Such an explanation leaves us exactly where we started. 



