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 generally takes place in such a manner that the substrate is decomposed into simpler indifferent constituents, after which the cell builds from the base upwards. In many cases this rebuilding is unnecessary. Such is the case when the cell only requires the energy contained in the absorbed substance. As soon, however, as substances are required as vital units of the cell, then they have to be adapted to the whole structural plan in all its minutest details. This is also the case when secretory substances, having a characteristic structure and a specific action, are to be formed.

Ye know of unicellular organisms which produce their body substance from very simple elements indeed. Thus, we know of organisms which produce their cell plasma from carbonates, nitrates, water, and salts. Others can draw their nitrogenous supply from any substance which will supply them with ammonia. Others, again, make use of the free nitrogen of the air. There are, however, even amongst unicellular organisms, some that are very fastidious and will only thrive in the presence of certain peptones. Others even require certain forms of protein from which to obtain their derivatives.

An exhaustive study of the sources of nitrogen