The manner of union between the cells in the most primitive _living colonies_, whether vegetable or animal, is analogous to that followed in the segmentation of the ovum in its ontogenetic (_i.e._, individual) development.
But the _manner of construction_ differs notably, as between _animal_ and _vegetable_ cells.
Vegetable cells, on the one hand, have a resistant and strongly protective membrane; animal cells, on the contrary, have either a very thin membrane or none at all. Vegetable cells, as though made _venturesome_ by their natural protection, proceed to invade their environment in colonies--in other words, the cells dispose themselves in series of linear ramifications--witness the formation of primitive algæ; and analogously the expansion of the higher types of vegetation into their environment, with branches, leaves, etc. And just as though the vegetable cell acquired self-confidence because it is so well protected, it becomes stationary and strikes its roots into the soil.
To this same fact of cellular protection must be attributed the inferior sensibility and hence the permanent state of obscured consciousness in vegetable life.
This protection against the assaults of environment, and the consequent lack of sensibility, constitute from the outset an inferior stage of evolution.
Animal cells have an entirely different manner of forming themselves into colonies; acting as though they were _afraid_, they group themselves in the form of a little sphere, enclosing their environment within themselves, instead of reaching out to invade it; and subsequent developments of the animal cell consist in successive and complex _invaginations_, or formations of layers, one within another--instead of ramifications, after the manner of vegetable cells.
Accordingly, if we advance from that primitive animal type, the volvox, consisting of a simple group of cells arranged spherically, like an elastic rubber ball, to the coelenterata, we meet with the phenomenon of the first invagination, producing an animal body consisting of _two layers_ of cells and an internal _cavity_, communicating with the exterior by means of a pore or mouth. The two layers of cells promptly divide their task, the outer layer becoming _protective_ and the inner _nutritive_; and in consequence of their different _functions_, the cells themselves _alter_, the outer layer acquiring a tougher consistency, while the inner remains soft in order to absorb whatever nutriment is brought by the water as it passes through the mouth. In this way, there is a division of labor, such that all the external cells protect not only themselves, but the whole organism; while the internal cells absorb nutriment not only for themselves but for the others. This is the simplest example of a process that becomes more and more complex in the formation of higher organisms; in adapting themselves to their work, the cells become greatly modified (formation of tissues) and perform services that are useful to the entire organism. And at the same time, because of the very fact that they have been differentiated, they become dependent upon the labors of others, for obtaining the means of subsistence. Similar laws seem to persist even at the present day in the formation of _social organisms_, in human society.