The nucleus of these unicellular animals is often elongated (_e_), and shaped according to the general shape of the animalcule; but it is the same thing as the “nucleus” of all cells, whether of plants or animals—a denser “kernel” of protoplasm, limited by its own delicate sheath or membrane. It shows, like the cell-nucleus of ordinary cells, a special affinity for certain dyes, which do not stain the rest of the cell, so that it can be made very obvious and clear when the animalcule is killed by alcohol, picric acid, or other preservative solutions, and then stained; and it shows a curious breaking-up of its substance into thread-like fibres when the animalcule is about to divide into two—as is seen also in all cells when the regular process of division of one cell into two commences. The larger animalcules have enabled us to find out what are the special properties of the nucleus of cells, as contrasted with those of the rest of the protoplasm. The trumpet animalcule (Stentor) is a single cell, and though only one-thirtieth of an inch long, is large enough to be cut into pieces by very skilful use of a fine blade. It is found that, if we cut the Stentor into four or five bits, all continue to “live”; that is to say, to swim about by the vibration of the hair-like cilia on their surface. But those bits which have no part of the nucleus in them die after a few hours. They cannot take nourishment nor grow. On the other hand, all the bits which comprise a slice of nucleus commence to contract, and shape themselves like the original Stentor, then form a mouth, and take nourishment, and grow up to be fully-sized, complete Stentors—animalcules like that by the cutting-up of which they were formed. This and similar experiments are held to prove that the processes of nutrition, growth, and production of specific form are dependent on the nucleus. In its absence, you may have contractility and active movement for a time, but no repair, no building-up of new material, no directed or seemingly “purposive” movement. Such movements, viz., advance in one direction, arrest, hesitating, or exploring movement to the right and left, followed by rapid retreat or advance in a straight line, are often exhibited by these minute animalcules, and cannot be distinguished in character from those, say, of a fly or even of a mouse.