348 THE AMERICAN NATURALIST [Vol. LIII 



to a close only through the attainment of the form which 

 is typical for the species in question. Why should this 

 ultimate condition of equilibrium be the same whether 

 we start from an isolated blastomere, an irregular frag- 

 ment of a blastula or a normal egg? Does it not seem as 

 if the only constant feature in this case were the end 

 itself? In considering the behavior of a protozoan, the 

 stimuli may vary and the method of escape may vary, 

 but the organism itself is the same. The ^'equi-finality" 

 of the result— to use an expression of Driesch's— may be 

 attributed to this fact that we are dealing with the same 

 physico-chemical system, and one of the self-regulating 

 type. But w^hat of our various embryonic fragments? 

 Are they not obviously different physico-chemical sys- 



Now, after all, the difference between this case and 

 that of a regenerating limb or lens appears to me to be 

 only one of degree. The distinctions relate (1) to the 

 stage in development at which the injury is inflicted, and 

 (2) to the proportional part of the organism which is left 

 to reconstruct the remainder. 



1. As regards the first point, we must suppose that at 

 each stage of ontogeny such a state of physiological bal- 

 ance is normally maintained as is appropriate to that 

 particular stage. That the multiplication and differen- 

 tiation of certain cells is profoundly influenced by the 

 presence or absence of other cells is one of the assured 

 results of experimental embryology. One need only cite 

 the difference between the development undergone by an 

 amphibian blastomere which is totally detached at the 

 two-celled stage, and that of the blastomere whose part- 

 ner has been injured by a needle-prick and left in po- 

 sition. 



Thus we have as much right to assume for tlie l)l;istula 

 as for the adult animal that any disturbance of metabolic 

 balance will be followed by varied responses, some of 

 which will tend to restore the balance normal to that 

 period. The fact that these responses are known to 

 differ radically, following the same type of operation, 



