cells, a forecast of the division of labour found amongst the organs of the higher forms of life. In the former case we have the cells as yet free, while in the latter they are combined into tissues. From this point of view we may look upon the symbiosis of heterogeneous species of cells as a first experiment in the building up of a cell state. The single cells are still independent and their duties multifarious. There is no strong bond uniting the organisms into any one "organ," and yet they depend upon each other for mutual support. Unicellular beings begin to organize themselves into combinations. Another step further and we arrive at cell complexes having definite functions, which we call organs. But even the most developed organisms, both of the animal and plant worlds, have relations with cells which stand outside the common organization. By means of micro-organisms the plant gains access to otherwise inaccessible sources of nitrogen, while by means of bacteria the animals make use of the important carbohydrate, cellulose. The bacteria convert the latter, within the intestines, into products which can be further decomposed by the ferments secreted by the glands.
In those organisms in which division of labour has been instituted amongst the cells, and particularly in those in which definite cells have closed in to form an alimentary canal, these are the only cells which
