/O CELL-DIVISION 



experiments suggest that the pathological mitoses in such growths 

 may be caused by the presence of deleterious chemical products in 

 the diseased tissue, and perhaps point the way to their medical 

 treatment. 



D. THE MECHANISM OF MITOSIS 



We now pass to a consideration of the forces at work in mitotic 

 division, which leads us into one of the most debatable fields of 

 cytological inquiry. 



i. Function of tJie AmpJiiaster 



All observers agree that the amphiaster is in some manner an 

 expression of the forces by which cell-division is caused, and many 

 accept, in one form or another, the view first clearly stated by Fol, 1 

 that the asters represent in some manner centres of attractive forces 

 focussed in the centrosome or dynamic centre of the cell. Regarding 

 the nature of these forces, there is, however, so wide a divergence of 

 opinion as to compel the admission that we have thus far accom- 

 plished little more than to clear the ground for a precise investigation 

 of the subject; and the mechanism of mitosis still lies before us as 

 one of the most fascinating problems of cytology. 



(a) TJie TJieory of Fibrillar Contractility. The view that has 

 taken the strongest hold on recent research is the hypothesis of 

 fibrillar contractility. First suggested by Klein in 1878, this hypoth- 

 esis was independently put forward by Van Beneden in 1883, and 

 fully outlined by him four years later in the following words : " In 

 our opinion, all the internal movements that accompany cell-division 

 have their immediate cause in the contractility of the protoplasmic 

 fibrillae and their arrangement in a kind of radial muscular system, 

 composed of antagonizing groups " (i.e. the asters with their rays). 

 "In this system the central corpuscle (centrosome) plays the part 

 of an organ of insertion. It is the first of all the various organs 

 of the cells to divide, and its division leads to the grouping of the 

 contractile elements in two systems, each having its own centre. 

 The presence of these two systems brings about cell-division, and 

 actively determines the paths of the secondary chromatic asters " 

 (i.e. the daughter-groups of chromosomes) " in opposite directions. 

 An important part of the phenomena of (karyo-) kinesis has its effi- 

 cient cause, not in the nucleus, but in the protoplasmic body of the 

 cell." 1 This beautiful hypothesis was based on very convincing 



1 '73- P- 473- a '87, p- 280. 



