lOO 



CELL-DIVISIOX 



D. The Mfxhamsm of Mitosis 



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

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



cytological inquiry. 



I. Function of the Aviphi- 

 astcr 



CZ- 



in. z. 



v-ac 



a.c 



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 dv- 

 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 ])recise in- 

 vestigation of the subject ; 



Fig. 48. — Slightly schematic figures of dividing eggs ^ . ' . 



o{ Ascaris, illustrating Van Bencdcn's theory of mitosis, and the mccnanism Ot mi- 



[Van Beneden and juLiN.] tosis Still lies before us as 



A. Early anaphase; each chromosome has divided qj-j^. ^f ^\^^^ moSt fascinating 

 into two. B. Later anaphase during divergence of the ^ . 



daughter-chromosomes, a.c. Antipodal cone of astral problems 01 CytOlOgy. 

 rays; f.s. cortical zone of the attraction-sphere ; /. inter- (^(ji\ 'J'Jw TllCOiy of Fl- 



zonal fibres stretching between the daughter-chromo- u,.;i] ^,. r ...f,- , -tUi t^, HTUo 



somes; m.z. medullary zone of the attraction-sphere; ^^ '^^^^' Coutl Octlllty . —^\x<. 



p.c. principal cone, forming one-half of the contractile vicVV that haS taken the 



spindle (the action of these fibres is reenforced by that of cf-rnno'est hold On rCCCnt 

 tlie antipodal cone) ; s.e.c. subequatorial circle, to which t> ^ _ 



the astral rays are attached. 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 outlined 



1 ' 



73. P- 473- 



