THE LIVING SUBSTANCE. 



53 



that area. This is true whether the area is one stably organized 

 for contractile activities or vegetative in its habit. ^ Even in 

 strongly contracted masses and areas having a marked struc- 

 ture of Biitschli there are numberless local disturbances of the 

 interalveolar substance, even transposition of this which is yet 

 responsible for the structure and functional activity of the area 

 in gross. After organized function throughout the area, por- 

 tions of the contracted substance relax, but the characteristic 

 organization of the structure remains marked and stable in the 

 whole area. This shows certainly that not all of the continu- 

 ous substance is necessarily involved in maintaining contrac- 

 tion of any area, on the basis of a given structure. 



In the few Metazoan organisms whose development I have 

 watched, there is a more or less gradual formation, peripheral 

 to the mass, of an area which can be grouped with typical ecto- 

 sarc alone. And for each so called cell mass, or nucleated and 

 pellicularly separated subdivision of the general mass, there is 

 also such a layer.^ As the Metazoan substance, in its life- 

 rhythm as organism, approaches an adult stage, there gradually 

 comes to be by successive reorganizations of its elements, an 

 area of true ectosarcal type in gross. This has been called the 

 ectoderm, and is formed of "cells," to the general substance of 

 each of which the same typical structure may be extended in a 

 variety of ways. Of such Metazoan ectosarc are made, as a 

 rule, all areas of the mass, or organism, which as organs corre- 

 spond in function with true ectosarc in Protozoa; that is, which 

 act as intermediators between the mass and its external environ- 

 ment. True ectosarcal formations of the Metazoan are, how- 

 ever, by no means confined to such cells; but most, or perhaps 

 it may even be said, all other cells form to some extent such 

 areas of their substance.^ Nor can these be held identical in 

 any way with that purely physical ectosarc of artificial foams, 

 since in their origin the living substance seems to be neither 

 dominated wholly, nor inhibited, by those physical conditions 

 which Biitschli has shown us do, in artificial foams, produce, 

 or forbid production of, such areas, 



^ See Activities — filose ; also Contractility. 



^ See Activities, starfish and sea-urchin development. 8 ggg Striation. 



