500 C. M. Child 



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

 embryonic development is universally applicable. There are 

 certainly many cases in which the terminal portions attain or 

 approach their final condition of functional activity before the 

 basal parts are formed, and in a considerable number of cases 

 also the basal parts are replaced incompletely or not at all. In 

 fact it seems to me that such cases might be expected to occur, 

 for in a correlated system the conditions for the regulatory forma- 

 tion of non-terminal regions must, at least sometimes, be largely 

 dependent on the existence of typical functional conditions in 

 terminal parts. If conditions in the terminal parts are more im- 

 portant than those in the old parts as determining factors in the 

 differentiation of intermediate parts we should expect to find the 

 intermediate parts differentiating later than the terminal parts, 

 but if, on the other hand, conditions in the old parts are the chief 

 determining factors, differentiation might occur wholly in the 

 centrifugal direction. 



Moreover, although I agree with Holmes that the absence of 

 visible differentiation does not necessarily imply absence of physi- 

 ological differentiation, I am incined to believe that the direction 

 of progression of visible differentiation is not without significance 

 as an indication of the direction of progression of physiological 

 differentiation. In other words, while the absence of visible 

 differentiation proves little or nothing with regard to physiological 

 differentiation, its presence may prove something. I think it 

 probable therefore that in som.e cases the regenerating part is not 

 differentiated as a whole, but that its various regions are deter- 

 mined successively in one direction or the other: in other cases 

 it may perhaps be differentiated as a whole. It would appear 

 that none of these possibilities conflict with either Holmes' 

 hypothesis or my own. 



undoubtedly open to misinterpretation and should have been qualified, for I was well aware at the time 

 it was made that centrifugal differentiation occurred in various cases. In fact, I had shown in earlier 

 papers (e. g., Child, '04b) that the differentiation of the intestine in regenerating parts of Leptoplana 

 is apparently centrifugal. However I take the present opportunity to make acknowledgments to 

 Holmes and Zeleny for calling my attention to this misleading statement, and also to make clear my 

 real position in the matter, which is that differentiation may occur in either direction or in both accord- 

 ing to conditions in the particular case. 



