SOME MODERN VIEWS OF THE CELL. 607 



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 out- 

 side of the nucleus less specialization than to the latter, there ap- 

 pears to be a certain portion of it which has a special role. As 

 early as 1883 the Belgian zoologist, 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 mak- 

 ing 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 pre- 

 pared 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 ani- 

 mals consist of single cells, while others are built up of mil- 

 lions 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 Bacteria, 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- 



