_The Nucleus_.—Every cell capable of living, growing, and multiplying, possesses a _nucleus_ of constitution very analogous to the cellular mass which surrounds it. The anatomical elements in which no nucleus is found, such as the red globules of blood in adult mammals, are bodies which are certain, sooner or later, to disappear. There is therefore no real cell without a nucleus, any more than there is a nucleus without a cell. The exceptions to this law are only apparent. Histologists have examined them one by one, and have shown their purely specious character. We may therefore lay aside, subject to possible appeal from this decision, organisms such as Haeckel’s _monera_ and the problem of finding out if bacteria really have a nucleus. The very great, if not the absolute generality of the nuclear body, must be admitted. It hence follows that there is a nuclear protoplasm and a nuclear juice, just as we have seen that there is a protoplasm and a cellular juice. What was just said of the one may now be repeated of the other, and perhaps with even more emphasis. The nuclear protoplasm is a filamentary mass sometimes formed of a single mitome or cord, folded over on itself and capable of being unrolled. The mitome in its turn is a string of microsomes united by the cement of the linin. These are the same constituent elements as before, and the language of science distinguishes them one from the other by a prefix to their name of the words _cyto_ or _karyo_, which in Greek signify cell and nucleus, according as they belong to one or the other of these organs. These are mere matters of nomenclature, but we know that in the descriptive sciences such matters are not of minor importance. We have just indicated that in a state of repose,—that is to say, under ordinary conditions,—the structure of a nucleus reproduces clearly the structure of the cellular protoplasm which surrounds it. The nuclear essence is best separated from the spongioplasm. It takes more clearly the form of a filamentary thread, and the filaments themselves (mitome) show very thick chromatic granulations, or microsomes, connected by the linin. At the moment of reproduction of the cell these granulations blend into a stainable sheath which surrounds the filaments, and the latter dispose themselves so as to form a single thread. This chromatic filament, which has now become a single thread, is shortened as it thickens (_spireme_); it is then cut into segments, twelve or twenty-four in the case of animals and a larger number in the case of plants. These are _chromosomes_, or _nuclear segments_, or _chromatic_ loops. Their part is a very important one. They are constant in number and permanent during the whole of the life of the cell. Let us add that the nucleus still contains accessory elements (nucleoli).