Growth and Kegeneration 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 
will 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 for 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 living 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- 
