THE VITAL PHENOMENA OF THE CELL 179 



Gegenbaur; Haeckel V. 4b; van Beneden, etc.), the nucleus 

 was supposed to take an active part in the process of cell-division, 

 and, at the commencement of it, to become elongated and con- 

 stricted at a point, corresponding with the plane of division which 

 is seen later, and to divide into halves, which separate from one 

 another and move apart. The cell body itself was supposed to 

 become constricted, and to divide into two parts, in each of which 

 one of the two daughter-nuclei formed the attraction centre. 



Each of these theories, so diametrically opposed, contains a 

 grain of truth, although neither describes the real process, which 

 remained hidden from the earlier histologists, chiefly on account 

 of the methods of investigation used by them. It is only during 

 the last two decades, that our knowledge of the life of the cell has 

 been materially advanced by the discoveries made by Schneider 

 (VI. 66), Fol (VI. 18, 19), Auerbach (VI. 2a), Biitschli (VI. 81), 

 Strasburger (VI. 71, 73), O. and R. Hertwig (VI. 30-38), Flem- 

 ming (VI. 13-17), van Beneden (VI. 4a, 4b), Rabl (VI. 53), and 

 Boveri (VI. 6, 7). These discoveries have revealed to us the 

 extremely interesting formations and metamorphoses, which are 

 seen in the nucleus during cell-division. These investigations, to 

 which I shall have occasion to refer frequently in this section, 

 have all pointed to the same conclusion, that the nucleus is a 

 permanent and most important organ of the cell, and that it 

 evidently plays a distinct role in the cell life during division. 

 Just as the cell is never spontaneously generated, but is produced 

 directly by the division of another cell, so the nucleus is never 

 freshly created, but is derived from the constituent particles of 

 another nucleus. The formula, " omnis cellula e cellula^ might be- 

 extended by adding " omnis nucleus e nucleo " (Flemming VI. 12), 



After this historical introduction, we will consider more in detail, 

 first, the changes which take place in the nucleus during division, 

 and next, the various methods of cell multiplication. 



II. Nuclear Division. The nucleus plays an important and 

 most interesting part in each process of cell-division. Three 

 methods of nuclear reproduction have been observed : indirect, or 

 nuclear segmentation, direct (Flemming), or nuclear fission, and! 

 endogenous nuclear formation. 



1. Nuclear Segmentation. Mitosis (Flemming). Karyokinesis 

 (Schleicher). The phenomena which occur during this process 

 are very complicated ; nevertheless they conform to certain laws 

 which are wonderfully constant in both plants and animals. 



