are explained by Geometry. 15 
and to preserve them as cells, they will require to have a more 
stable structure than cells in general. In a word, on surfaces 
exposed to an ambient medium which is favourable to life, an 
epithelium is called for. 
Vessels, 
Nevertheless, in those regions in which the ambient medium 
presses most urgently upon the cellular mass, we are to ex- 
pect the cells to open and to admit the ambient medium to 
the interior. And thus, on the single principle on which the 
whole of this paper has been reasoned, we shall have in cel- 
lular masses, when the conditions of existence are favourable 
to the deployment of life, intercellular passages, lacune, and 
vessels which, when the external pressure is local, and the 
cells easily transformable, will be conical as they retire from 
the region of maximum pressure, and tend to ramify. . 
A Dermal System. 
It is easy to perceive, however, that the limit of this process 
of vessel-formation, and of the admittance of the ambient 
medium to the interior of a cellular mass, is the complete 
solution of the cells which first formed the interior of that 
mass, and the reduction of the entire orgasm to a dermal body, 
pervious, by one or more openings, to the ambient medium. 
Viscera, &fc. 
But as fast as this process of cell digestion proceeds in the 
interior of the mass, a peculiar liquid must be generated 
therein—a liquid composed of cell-material. Hence, we are 
only to expect that, within the persistent dermal envelope, 
new cellular structures will be formed, and differentiation 
proceed, special organs being modelled according to the type 
of the species. Thus far we can go on a single principle—and 
that, an element of pure geometry. But to explain the multi- 
plication of cells, as also the morphology of compound forms 
generally, another law must be invoked—tTHE LAw oF ASsIMI- 
LATION. This, however, it forms the principal theme of a 
special work* to unfold, and I need not touch upon it here. 
* The First Lines of Science Simplified, and the Structure of Molecules 
attempted. Sutherland and Knox. Edinburgh, 1860. 
