IOC 



CEI.I.-DIVISIOX 



D. The Mfxhanism of Mitosis 



W'c now jxiss to a consideration of the forces at work in mitotic 

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



cytological inquiry. 



I. Function of the AvipJii- 

 aster 



CZ 



niz. 



ac 



a.c 



First suggested by 

 ently put forward 



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,^ 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) Tlic Theory of Fi- 

 brillar Contractility. — The 

 view that has taken the 

 strongest hold on recent 

 research is the hypothesis 

 of fibrillar contractility. 

 Klein in 1878, this hypothesis was independ- 

 bv Van Beneden in 1883, and fully .outHned 



Fig. 48. — Slightly schematic figures of dividing eggs 

 of Asciiris, illustrating \'an 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 ; /'. inter- 

 zonal fibres stretching between the daughter-chromo- 

 somes; in.z. medullary zone of the attraction-sphere; 

 f.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 ravs are attached. 



1 '7J> 1 



P- 473- 



