Studies 071 Regulation. 281 



The phenomena of regulation in the broadest sense constitute 

 at present one of the most important and promising fields for work. 

 Within the last few years many of our conceptions regarding devel- 

 opment have been profoundly modified by the results of experi- 

 mental work along these lines and there can .be little doubt that 

 they are destined to still greater modification. Whatever modifies 

 our theories of development must alter our ideas regarding inherit- 

 ance and the nature of the germ-cell, both problems which are 

 receiving much attention from morphologists. The physiological 

 investigation of development will probably aff^ord in future far 

 more numerous and exact data regarding the nature of inheritance 

 and other fundamental problems of biology than any other field 

 of research. The phenomena of regulation differ from those of 

 embryonic development as the conditions differ in the two cases. 

 Many parts of the field are accessible even at present to experi- 

 mental methods and there can be no doubt that in future it will be 

 possible to extend control of them much farther. 



Roux ('95) maintains, however, that two distinct categories ot 

 development "typical" and "regulatory" must be recognized and 

 that the mechanisms concerned in the formation of a given struc- 

 ture in typical development may be different from those which 

 come into play in regulation. Hypotheses of this kind only 

 increase instead of diminish our diflficulties and, moreover, they 

 are based on theoretical considerations and not on observation 

 and experiment. If we admit instead that form and structure are 

 results of reactions to conditions it is evident that changed condi- 

 tions may change both the processes and the results. Regeneration 

 of a part removed may differ widely from the ontogenetic develop- 

 ment of the same part, but, as I have attempted to show in this and 

 preceding papers on Leptoplana (Child, '04a, '04b, '04c) the con- 

 ditions to which the regenerating tissue is subjected are different 

 from those to which the part is subjected in ontogeny; in the one 

 case the tissue arising from the cut surface is connected with a 

 fully developed part, in the other all parts are developing together. 

 It is to be expected therefore that the course of regeneration will 

 be briefer than that of ontogeny and, moreover, that it will differ 

 in various respects, according as particular conditions differ. 

 Notwithstanding these differences, indeed often because of them, 

 the phenomena of regeneration are of great importance in the 

 physiology of development. But other methods of regulation 



