THE DYNAMIC FACTOR IN REGENERATION. 275 



statement a meaning for those who believe no suggestion to be 

 of value unless it refer the problem to ordinary properties of 

 inorganic bodies, I suggested that osmotic pressure might be the 

 cause of the tension differences in the parts. This was an 

 unnecessary concession. The behavior of fluid crystals (accord- 

 ing to Lehmann) shows that the formative changes can be 

 accounted for on the basis of a tension exhibited by the mole- 

 cules of the substance of the crystal. While the organism may 

 not be put down as a fluid crystal, still we see that physical 

 properties other than osmotic pressure and surface tension may 

 play an all-important role in form-changes. It may be that a 

 similar property is the cause of the formative changes in the 

 organism. In any case, the facts that I had in mind suggested 

 that tension of some sort is an important dynamic factor in 

 development, perhaps the important factor. The facts still seem 

 to me to indicate some such relation between the parts, and no 

 one regrets more than I that we cannot "explain" the results 

 even if my suggestion prove to be in the right direction. 



Still later a consideration of certain facts of development led to 

 the suggestion that two known properties of the organism con- 

 tractility and irritability also play a very important role in 

 embryonic and regenerative development. I shall not attempt 

 to review here the argument which led to this point of view. 

 How far and in what sense contractility and irritability are better 

 expressions of the tension hypothesis, it is not easy to state. 

 So far as contractility is concerned Lehmann's recent important 

 paper on " Scheinbarlebende Kristalle " l shows the possibility at 

 least of referring this property also to a condition of molecular 

 tension. We are still too ignorant of the physical basis of 

 irritability to make speculations in this direction profitable, but it 

 may be well not to lose sight of this property of living matter in 

 our attempts to analyze further the problem of development. 



STEREOMETRY OF THE BIOPLASM. 



Polarity implies difference in one direction. Every student of 

 regeneration knows that in all three dimensions of space the same 

 factor is present. Polarity is therefore only a part of the problem, 



l io/. Centralb., XXVIII. , 1908. 



