lOO 



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. Functioji 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 Yo\} 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 



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 11 ^f CVtolo^-V 



daughter-chromosomes, a.c. Antipodal cone of astral prODlCn^S Ui ^y LUiu^jy. 

 rays ; c.z. cortical zone of the attraction-sphere ; /. niter- 

 zonal fibres stretching between the daughter-chromo- 

 somes; OT.2. 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. 



(rt) The TJieoiy of Fi- 

 brillar 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 hypothesis was independ- 

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



^ '73. P- 473- 



