Physiological Basis of Restitution of Lost Parts 499 



like that of the missing part. That is only one step. We have 

 then to account for the numerous coordinated differentiations 

 that take place as the part develops toward the base. * * * The 

 fact that, with few exceptions, such as the failure to regenerate the 

 intermediate segments of the appendages, etc., the whole organ, 

 nothing more nor less, is regenerated, and forms a congruent union 

 with the basal part, is indicative of close interaction of the various 

 parts of developing organs with the body of the organism at all 

 stages of the process. 



"I am inclined to think that neither centrifugal nor centripetal 

 differentiation, expresses the entire truth of the matter, but that 

 the new part differentiates as a whole, much as organs do in embry- 

 onic development, and at all times in intimate functional relations 

 with the old part, differentiation becoming accelerated in one part 

 or another, according to special conditions" (Holmes '07, pp. 

 428, 429). 



As regards most of these points my position does not differ very 

 widely from that of Holmes. My criticism of his analysis of the 

 case of Planaria was directed primarily, not at his hypothesis in 

 general but merely at his failure to consider the actual facts in 

 that case. I see no reason why the occurrence of differentiation 

 in either direction or in both should constitute a fatal objection 

 to his hypothesis or to my own, for such differences are merely 

 incidental and depend on the conditions in individual cases. 

 When my criticism was written the experimental data seemed to 

 indicate that visible differentiation in the centripetal direction 

 was the general rule, though by no means without exceptions, and 

 since Holmes did not in his first paper attempt to account for 

 this fact in any way, his hypothesis was open to criticism. I cer- 

 tainly had no intention of maintaining that differentiation must 

 in all cases proceed centripetally, since at that time various cases 

 were known to me in which visible differentiation proceeded centri- 

 fugally.^ I do not believe however that Holmes' suggestion that 



1 In his discussion of the direction of differentiation in the antennule of Mancasellus, Zeleny ('07, p. 

 335) says: "Child has recently expressed the opinion that differentiation must in every case proceed 

 from the tip toward the base and in no other way." My actual statement was that differentiation from 

 the tip toward the base is "a general ruel in cases of regeneration." This statement as it stands is 



