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THE AMERICAN NATURALIST [Vol. XLIV 



mation in development and regeneration which it brought 

 to light, but because it calls into question the conception 

 of the determined specificity of tissues. And in point of 

 fact, the problem of the relation between regeneration 

 and ontogeny ultimately resolves itself into this wider 

 problem of the specificity of tissues. In the crayfish 

 and hermit crab cells, which under ordinary conditions 

 of development never give rise to muscles, apparently 

 have the potentiality to do so under certain circum- 

 stances. 



The exceptional cases, alluded to above, are not of the 

 kind that would tend to corroborate the rule. On the con- 

 trary, they tend to turn the whole question topsy-turvy. 

 It rests with the adherents of the view that regeneration 

 and ontogeny are parallel processes to bring forth a 

 creditable explanation of these facts to fit them into their 

 theory, for as long as they remain undisputed facts they 

 must likewise remain the unapproachable stronghold of 

 skepticism. 



Davydoff in his recent monograph traced with great 

 skill and painstaking care the history of each regenera- 

 ting organ of Ptychodera. This laborious investigation 

 led him to conclude that "the study of regeneration in 

 Enteropneusta entirely corroborates the hypothesis that 

 ... all organs and tissues regenerate from elements of 

 the same germ-layers from which they also developed in 

 ontogeny" (p. 78). 



This conclusion is adopted "even though at times 

 there is no complete resemblance to the ontogenetic 

 process" (ibid.). 



The reservation is specially noteworthy since in 

 another place we read the following: 



