STRUCTURE AND DIVISION OF ORGANIC CELL. 817 



the cell. Around the cell-wall, in this plane, is seen a faint ring, which 

 pushes inward, and develops into a new wall of cellulose. It extends 

 into the space between the two equatorial plates, and continues to 

 grow inward until the center is reached, when it forms the dividing 

 wall of the two new cells. As it grows, the spindle, which had pre- 

 viously been swelling outward, begins to contract, until it becomes a 

 narrow series of lines, reaching from the poles to the new cell-wall. 

 Meanwhile the polar masses secrete new membranes, and assume the 

 condition of nuclei of the new cells. So rapid is the process that the 

 nucleolo-nucleus often again divides ere the nucleus has completed its 

 division, and the nucleoli again divide ere the new cells are formed. 

 Hence the new nucleus often has two nucleoli. After complete divis- 

 ion the lines of the nuclear spindle are still apparent. They may, by 

 splitting, give rise to the fibers of the new cells. 



Such is a recent description of the process of division in the plant- 

 cell. In the cells of some plants, however, there is a preliminary step 

 of change which does not appear in Spirogyra. In these cases division 

 begins with a massing of the nuclear contents in the equatorial region. 

 The nucleus has a spindle-shape, with a dark mass in its center, and 

 clear areas reaching to its poles. This mass splits, and its two halves 

 retreat to the poles. The further steps of division are as above. 



Thus, so far as now appears, the process of cell-division in the 

 plant is closely analogous to, though not identical with, that of the 

 animal. It seems, indeed, a more primitive stage of the phenomenon. 

 The division of the nucleolus, so marked in the plant, has not been 

 observed in the animal, and may be, in the latter, suppressed or has- 

 tened, like many of the developmental changes in the higher animals. 

 On the other hand, the peculiar movements of the chromatin-fibrils of 

 the animal cell have no direct counterpart in the plant. They seem to 

 present a distinct step forward in cellular evolution, and yield the idea 

 that the animal cell is a more advanced organism than that of the 

 vegetable. It certainly seems to hasten or suppress embryo changes 

 which are well marked in the latter, and to clearly display advanced 

 stages of development which are only vaguely outlined in the latter. 



There is another cell-theory extant to which some allusion must be 

 made, as it indicates a final stage in cell-evolution in advance of that 

 here indicated. It is known that in many cases elongations of the 

 fibrous network extend outward from the cell. These have been seen 

 in epithelial cells, joined so as to form a connecting link between two 

 cells. It is well known also that numerous delicate fibrils extend 

 beyond the walls of nerve-ganglion cells, probably as outer continua- 

 tions of the internal network. It is supposed that these fibrils aggre- 

 gate into bundles, and that thus the nerve-fibers, which run to all parts 

 of the surface of the body, originate. This seems to be the cell con- 

 nection of the sensory nerves, while the motor nerves leave the cells 

 each as a single fiber. The nerve terminations of muscles present a 

 Toi,. XXVI. — 52 



