INTERNAL FACTORS OF REGENERATION IN ANIMALS 



the facts themselves, and may withdraw attention from the real solu- 

 tion of the problem. 



Bonnet, who first proposed the hypothesis of specific stuffs, went 

 further and assumed also that they move in definite directions in the 

 body, the head-stuff flowing forward and the tail-stuff flowing back- 

 ward. It was necessary to assume definite movements of the stuffs 

 in order to account for the development of the head at the anterior 

 end of a piece and of a tail at the posterior end. In cases of hetero- 

 morphosis of the sort described above, these stuffs, if they brought 

 about the results, would have to move in opposite directions from those 

 assumed in the hypothesis ; or else that part of the hypothesis that 

 postulates the movement of the substances must be dropped, and in its 

 place there must be substituted the idea of the excessive amount of 

 such substances in the ends accounting for the heteromorphosis. An 

 hypothesis that must be changed in this fundamental way to explain 

 both classes of facts cannot be given very serious consideration. Of 

 these possible ways in which it has been 

 attempted to account for the phenomenon 

 of heteromorphosis, the first one suggested 

 seems to me simpler and more probable, but 

 which organs are to be made responsible 

 for the result cannot at present be stated. 

 The fact that both Bardeen and I have ob- 

 tained heteromorphosis in planarians in 

 other regions than in the head indicates 

 at least that other factors than the presence 

 of head tissues or of head substances may 

 bring about the development, and if it can 

 be discovered what produces the result in 

 regions remote from the head we may be 

 in a position to explain the result in the 

 head region in the same way, although it may 

 be, of course, that the same result may be 

 brought about by different factors, when the 

 internal conditions are somewhat different. 



Another phenomenon connected with 



the polarity of a piece is shown by Ceri- FIG. 17. - After vTigt. pianarian 

 anthns mcmbmnaccons. When a triangular 

 piece is cut from the side of the body, a half 

 circle of tentacles appears around the lower 

 edge of the cut, as shown in Fig. 15, C. 

 The presence of a free distal edge on the 

 lower side of the opening is a sufficient 

 stimulus to call forth the development of tentacles. 



with three oblique cuts at side 

 The most anterior cut (left 

 side), directed forward, pro- 

 duced a tail. The one on the 

 right side, directed backwards, 

 produced a head. The most 

 posterior cut (left side) made 

 a head with pharynx, and also 

 a tail-like outgrowth. 



