Growth and Regeneration 281 



this is a head, and acting on the new material at the posterior 

 end determines that this is a tail. The centripetal influence is, 

 according to my interpretation, nothing more than the tension 

 of the outer layer of cells, and the pressure relations in general, 

 in the rounded dome-shaped mass of new materials. In this 

 way we can give a formal solution of the development of a head 

 in one case and of a tail in the other. 



Let us see whether the same hypothesis will explain the differ- 

 ent rates of growth of the posterior end according to the level 

 of the cut, as seen in the earthworm, salamander, and fish. A 

 growing region is present near, but not quite at, the tip of the tail. 

 From this region new material is continually being produced, out 

 of which the new part is differentiated. The way in which this 

 new part differentiates is determined by the pressure relation of 

 the neighboring parts. This pressure relation is the result of the 

 differentiation, with its concomitant pressure relations, that has 

 already taken place in the old part on the one side, and of the 

 tension of the new material of the tip on the other side. The 

 new part differentiates therefore into something that is less than 

 the former and more than the latter. In consequence there 

 w^ill be an ever decreasing stimulus and differentiation as the 

 new parts are formed, until finally no further stimulus for 

 growth and differentiation is present or is strong enough to act, 

 and the growth comes to an end. 



In some such way as this we can, provisionally at least, ac- 

 count for the difference in rate at different levels, since the rate 

 is determined by the pressure relations of the different parts, 

 and this pressure decreases as a stimulus from the middle 

 toward the posterior end. In principle this assumption refers the 

 changes that take place to a formative factor or factors. It as- 

 sumes that the differentiation ]or a given material is a response 

 to pressure relations. The nature of response is unknown, as 

 in all other cases where living material responds to external con- 

 ditions, but that Hving material possesses a power to respond 

 by differentiating to external agents, and even to pressure rela- 

 tions, is too well known to require demonstration. We are deal- 



