cell work—that of Strasburger on plant cells, which appeared in three editions from 1875 to 1880, and that of Flemming on animal cells, published in 1882. These authors showed that in both plants and animals cell division is ordinarily preceded by nuclear division. This latter process is not usually a mere constriction, but is, as we shall see, a highly remarkable and significant one, with a wonderful agreement in detail throughout both animal and vegetable kingdoms, so far as studied. The literature of the past fifteen years concerning this subject is of almost incredible volume, but it has all served to confirm the prime importance of the nucleus as an organ of the cell, and to show the correctness of Flemming's extension to the nucleus of the principle long before established for the cell, in writing "Omnis nucleus e nucleo."
While we attribute to the main mass of the protoplasm outside of the nucleus less specialization than to the latter, there appears to be a certain portion of it which has a special rôle. As early as 1883 the Belgian zoölogist, Van Beneden observed that certain tiny protoplasmic masses bear a definite relation to nuclear division, and he expressed his belief that these should be regarded as definite organs of the cell. This view has steadily gained ground, and, although they were not recognized in plant cells until 1891, when Guignard discovered them, on account of their minute size and of the technical difficulties connected with making them visible, their general occurrence and importance may now be said to be well established. These tiniest of the known organs of the cell are called centrospheres. Each consists of a central point surrounded by a mass of apparently homogeneous hyaline protoplasm.
Having now traced the development of our ideas, we are prepared to express our present conception of a typical cell as a mass of living protoplasm within which are differentiated a nucleus and one or two centrospheres. Many plants and animals consist of single cells, while others are built up of millions of these units of structure; but any organism is either an independent cell or an aggregation of cells more or less mutually interdependent. Some of the simplest unicellular organisms, like the Baderia, of whose work in the world we now hear so much, are so minute that no differentiation within the cell has been observed. But rapid improvement in methods of study and means for observation are steadily reducing the number of these. Let us try, then, to get an idea of the best-established facts and views concerning the activities of the cell, and as to the part played by each of its organs in these activities.
Most plant cells are inclosed, as we have seen, in a firm wall, usually composed of a substance known as cellulose. Animal cells are, as a rule, without a definite membrane, and it is not cer-
