2/6 T. H. MORGAN. 



and so far as it draws attention away from the whole problem it 

 seems best to substitute the term stereometry. 



Sufficient evidence has accumulated, I think, to show that 

 stereometry has a dynamic side in so far as it is a result of the 

 molecular factors that determine the relations of the parts to each 

 other. A question of fundamental importance here presents it- 

 self. If the formed substances at each level are the products of 

 the bioplasm, must not the bioplasm itself be stratified in nearly 

 the same sense ? It was this idea that I had in mind when I 

 wrote in 1906 : " If we imagine a stereometric network as a part 

 of the specialized structure, we must be prepared to admit that it 

 changes at each level as the structure changes. Therefore it 

 seems to me simpler to base our hypothesis of polarity on the 

 difference in differentiation itself, and not on an imaginary polar- 

 ized system associated with the living materials." But the point 

 I overlooked was that there is no need to suppose that a hetero- 

 geneous network of bioplasm exists because the visible structure 

 formed by it is different. The relation of the polarized material 

 to the ends of the material (indeed to all its directions) suffices to 

 account for the difference of level. In fact if the stereometry 

 rests on a dynamic and not a statical relation of the parts this is 

 the logical standpoint. 



It has been suggested that irritability may be related to the 

 dynamic factor of development. 



The effects of irritability at any level may be realised through 

 the cliemical changes inaugurated. TJiese chemical changes once 

 started may, if enzymatic, thenceforward continue (unless checked by 

 other chemical processes], independently of the factor that set them 

 going. 



