THE MECHANISM OF MITOSIS 



71 



K-ac 



cz- 



a.c 



evidence derived from the study of the Ascaris Qgg, and it was 

 here that Van Beneden first demonstrated the fact, already sus- 

 pected by Flemming, that the daughter-chromosomes move apart to 

 the poles of the spindle, 

 and give rise to the two re- 

 spective daughter-nuclei.^ 



Van Beneden describes 

 the astral rays, both in 

 Asca7-is and in tunicates, 

 as differentiated into sev- 

 eral groups (Fig. 34). One 

 set, forming the " principal 

 cone," are attached to the 

 chromosomes and form 

 one-half of the spindle, 

 and, by the contractions 

 of these fibres, the chro- 

 mosomes are passively 

 dragged apart. An oppo- 

 site group, forming the 

 " antipodal cone," extend 

 from the centrosome to 

 the cell-periphery, the base 

 of the cone forming the 

 "polar circle." These 

 rays, opposing the action 

 of the principal cones, not 

 only hold the centrosomes 

 in place, but, by their con- 

 tractions, drag them apart, 

 and thus cause an actual 

 divergence of the centres. 

 The remaining astral rays 

 are attached to the cell- 

 periphery and are limited 

 by a sub-equatorial circle. 

 Later observations indi- 

 cate, however, that this 

 arrangement of the astral rays is not of general occurrence, and that 

 the rays often do not reach the periphery, but lose themselves in the 

 general reticulum.- 



Van Beneden's general hypothesis was accepted in the following 

 year by Boveri {'88, 2), who contributed many important additional 



1 '83, p. 544- 



m.z. 



Fig. 34. — Slightly schematic figures of dividing eggs 

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

 [Van Beneden and Julin.] 



A. Early anaphase; each chcomosome 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. in- 

 terzonal fibres stretching between the daughter-chromo- 

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

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

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

 the antipodal cone) ; s.e.c. sub-equatorial circle, to which 

 the astral ravs are attached. 



