IQO 



BIOLOGICAL LECTURES. 



replaced only as much as is lost, it is clear that the new limb 

 (of a salamander) does not arise from a preexisting germ of a 

 limb. The wounded surface of the limb draws food material 

 to itself, and these new food molecules are organized into a 

 new limb. The arranging force is a molecular force that does 

 not work at a distance from the living substance of the stump 

 of the limb, but acts only in so far that the force draws the 

 food molecules into the sphere of influence of the molecules 

 in the stump itself, bringing them to a definite place, and in 

 this way laying down a new living layer over the cut end of 

 the limb. The way in which this new layer is organized 

 depends on the law of organization of the part, i.e., on the 

 chemical condition of the surface layer on which the new layer 

 is deposited. In a word, the condition of the new layer is the 

 necessary mathematical consequence of the condition of the 

 older regenerating layer. 



On the surface of the wound a knob of indifferent tissue 

 arises, but long before the first layer of the new material has 

 become fully formed (differentiated .-') it has in turn acted on 

 the succeeding layer, and this on the next, and so on, so that 

 all the layers appear almost at the same time ; but the proxi- 

 mal ones are somewhat nearer the definitive form than the 

 more distal ones. 



Pfliiger also points out that certain exposed surfaces can 

 organize new material only in one direction. In other words, 

 the organized surfaces of the body show a polarization, since 

 one side of a surface shows peculiarities not present in the 

 other. The direction of the polarization of the regenerating 

 surface is the cause of the direction of the new growth. F'or 

 this reason a portion of an animal cannot produce the entire 



animal. 



Pfliiger has not taken into account some of the most con- 

 spicuous facts of regeneration — facts known even at the time 

 at which he wrote. His explanation fails completely to account 

 for those cases where a piece of an animal, hydra, for example, 

 changes its form into a new small hydra. Moreover, Bonnet 

 had shown that when Lumbriculus is cut in two, only a few 

 new segments are added at the anterior end of the posterior 



