812 THE POPULAR SCIENCE MONTHLY. 



simplified that any skilled observer, with a good instrument and proper 

 care, may hope to successfully employ them. 



The matter of which an organic cell is composed is found to be not 

 simply a homogeneous, or slightly granular, mass of protoplasm. On 

 the contrary, it appears to be traversed in every direction by delicate 

 fibers, which form an intricate network or reticulum. The interstices 

 of this network are occupied by a fluid or semi-fluid substance of homo- 

 geneous appearance, though occasionally containing a few small gran- 

 ules. The reticulum occurs not only in the outer cell, but also within 

 the nucleus, and its fibers extend to and are apparently connected with 

 the nucleolus. Within this latter the fibrous formation has not been 

 traced. Some observers, indeed, declare that there is no nucleolus, but 

 that it is simply a node of the intersecting fibers. But this view is 

 not generally entertained, and late writers ascribe to the nucleolus an 

 important function. 



In the growth and division of the cell the nucleus appears to be 

 specially active, and the new doctrine known as karyokinesis relates 

 principally to the peculiar metamorphoses of the nucleus during cell- 

 division. Two phases of cell-life are now well marked. One of these 

 is an active stage, during which transformation of the cell-contents 

 rapidly takes place, and division follows. This is succeeded by a rest- 

 ing-stftge, in which all activity of the nucleus ceases, the fibers grow 

 less distinct, and a partly homogeneous condition results. This resting- 

 stage is, after a period, followed by a new period of activity. 



The behavior of the cell-contents, when treated with carmine or 

 other staining reagents, indicates that they are composed of at least 

 two distinct substances. During the resting-stage this does not appear, 

 for the whole cell takes the stain, though it deepens in the nucleus, and 

 still more in the nucleolus. But during the active stage only the fibers 

 take the stain, while the intermediate ground or basis substance re- 

 mains clear and transparent. From this difference in behavior the 

 name chromatin is proposed for the fibers, achromatin for the ground 

 substance. 



Flemming, one of the most skillful observers of these phenomena, 

 distinguishes two forms of division — the direct and the indirect. The 

 former — which may eventually prove to have no real existence — is a 

 direct separation, first of the nucleolus, then of the nucleus, and finally 

 of the cell. In the latter there is a peculiar metamorphosis of the nu- 

 cleus. Flemming, from observation of the cells of Salamandra, describes 

 the process as follows : 



The resting-nucleus possesses a faintly-defined reticulum of fibrils, 

 whose meshes hold a homogeneous ground substance, one or more nu- 

 cleoli, and occasionally a few small granules. Possibly these latter are 

 merely the nodes of the reticulum. When the active stage commences, 

 the membrane of the nucleus disappears, as also the nucleolus and the 

 granules. If the latter are merely nodes of the fibrillar network, we 



