100 



CELL-DIVISION 



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 the Amphi- 



aster 



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 first view 

 clearly stated by Fol, 1 that 

 the asters represent in 

 some manner centres of 

 attractive forces focussed 

 in the centrosome or dy- 

 namic centre of the cell. 

 Regarding the nature of 

 these forces, there is, how- 

 ever, so wide a divergence 

 of opinion as to compel the 

 admission that we have 

 thus far accomplished little 

 more than to clear the 

 ground for a precise in- 

 vestigation of the subject; 

 and the mechanism of mi- 

 tosis still lies before us as 

 one of the most fascinating 

 problems of cytology. 



(a) The Theory of Fi- 

 brillar Contractility. The 

 view that has taken the 

 strongest hold on recent 

 research is the hypothesis 

 of fibrillar contractility. 



a.c. 



cz. 



m.z. 



Fig. 48. Slightly schematic figures of dividing eggs 

 of Ascaris, illustrating Van Beneden's theory of mitosis. 

 [VAN BENEDEN and JULIN.] 



A. Early anaphase ; each chromosome has divided 

 into two. B. Later anaphase during divergence of the 

 daughter-chromosomes, a.c. Antipodal cone of astral 

 rays ; c.z. cortical zone of the attraction-sphere ; i. inter- 

 zonnl fibres stretching between the daughter-chromo- 

 soni'S; 1/i.z. medullary zone of the attraction-sphere; 

 p.c. principal cone, forming one-half of the contractile 

 spindle (the action of these fibres is reenforced by that of 

 the antipodal cone) ; s.e.c. subequatorial circle, to which 

 the astral rays are attached. 



First suggested by Klein in 1878, this hypothesis was independ- 

 ently put forward by Van Beneden in 1883, and fully outlined 



1 '73 P- 473- 



