CELL DIVISION. 9 



FORMATION AND REPRODUCTION OF CELLS. 



In the past, two sorts of cell formation have been recognized, namely 

 the spontaneous generation of cells, and the origin of cells through the 

 division of pre-existing cells. According to the theory of spontaneous 

 generation it was once thought that animals as highly organized as intes- 

 tinal worms came into existence from the fermentation of the intestinal 

 contents. After this had been disproved it was still thought that cells 

 might be formed directly from a suitable fluid, the cytoblastema. Some- 

 thing of the sort may have occurred when life began, and it is the expecta- 

 tion of certain investigators that conditions may yet be produced which 

 shall lead to the formation of organic bodies capable of growth and repro- 

 duction. At present, however, only one source of cells is recognized, 

 the division of existing cells. "Omnis cellula e cellula." A nucleus 

 likewise can arise only by the division of an existing nucleus. There 

 is no satisfactory evidence that a nucleus may be formed from non- 

 nucleated protoplasm. In cell division the nucleus divides first and then the 

 protoplasm, generally into two nearly equal parts. During the process 

 a special grouping and transformation of the nuclear substance occurs 

 in accordance with fixed laws. The ordinary mode of .cell division is 

 called mitosis or indirect division [karyokinesis] and the characteristic 

 groups of nuclear material are commonly known as mitotic figures. Mitosis 

 is arbitrarily but conveniently divided into three successive phases, the 

 prophase, melaphase, and anaphase, in which respectively the nuclear 

 material prepares for division, divides, and returns to its usual condition. 

 (The final stages of reconstruction are often grouped as a fourth phase, the 

 telophase.) In the details of mitosis there are considerable variations, 

 not only in different animals but also in different kinds of cells in a single 

 species. The account of the process which follows will apply only in a 

 general way to a particular case of cell division which the student may 

 be examining. 



MITOSIS. 



Prophase. The centrosome and nucleus approach one another until 

 the centrosome is close to the nuclear membrane where it lies surrounded 

 by the clear zone of archoplasm. The archoplasm contributes to the 

 formation of radiating fibrils which extend from the centrosome in all 

 directions, and are known collectively as the centrosphere [astrosphere]. 

 The two parts of the centrosome which had formed in the "resting stage" 

 by the division of one, are in the midst of the centrosphere. They move 



