that act on fats, carbohydrates, nucleo-proteids, nucleic-acids, phosphatides, and so on. Everything points to the fact that the cell has agents at its disposal which render it capable of splitting up, into their simplest units, all the complicated substances which are brought to it, or which it itself builds up. In favour of such a view, we may more particularly cite, besides the direct proof of the existence of ferments, the observation that in the metabolism of the cell all the units, out of which the complicated nutritive substances and the components of the cells are built up, are found to occur.
At the present time there is no doubt that an important part of the metabolic processes of the cells is furthered by ferments. In general, we may say that complicated substances are hydrolytically reduced in stages until the simplest structural units are formed. Once the latter appear, then the further reduction continues in stages, through various intermediate products, right down to the end-products of metabolism, or else the resulting products of decomposition form the starting-point for new syntheses. From these products very varied links are forged between very different groups of substances.
It is thus proved that, in a certain sense, each separate cell of the body is capable of digestion. This holds particularly for the white and red blood corpuscles; even the platelets are able to produce hydro-
