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If in its extension to contain the new formations within it the embryo-sac remains narrow, endosperm formation proceeds upon the lines of a cell-division, but in wide embryo-sacs the endosperm is first of all formed as a layer of naked cells around the wall of the sac, and only gradually acquires a pluricellular character, forming a tissue filling the sac.
From coenobia or social unions of these afterwards arose the lowest histones, multicellular plants and animals. By the sure help of the three great empirical "records of creation," palaeontology, comparative anatomy, and ontogeny, the history of descent now leads us on step by step from the oldest Metazoa, the simplest pluricellular animals, up to man.
Among pluricellular organisms man is of course supreme. He is the one form of animal life that is most highly differentiated. Knowing what you now know of microscopic anatomy, you cannot hold to the simple idea that the human body is a single life-unit. This is the naïve belief that is everywhere current among men today.
The pluricellular organism is nothing more nor less than a later development, a confederated association of unicellular organisms. Mark the development of such an association. Originally each separate cell performed all the functions of a separate life. The bonds that united it to its fellows were of the most transient character.
As the result of such differentiation, the pluricellular organism, as it comes ultimately to be evolved, is composed of many different kinds of cells. Each has its special function. Each has its field of labor. Each lives its own individual life. Each reproduces its own kind. Yet all are bound together as elements of the same "cell society" or organized "cell state."
And, finally, Verwörn and Binet have found in a single living cell manifestations of the emotions of surprise and fear and the rudiments of an ability to adapt means to an end. Let us now consider pluricellular organisms and consider them particularly from the standpoint of organic evolution.
All living organisms consist, then simply of cells. Those consisting of but one cell are termed unicellular; those comprising more than one cell are called pluricellular. The unicellular organism is the unit of life on this earth. Yet tiny and ultimate as it is, every unicellular organism is possessed of an independent and "free living" existence.
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