THE MECHANISM OF MITOSIS 



evidence derived from the study of the Ascaris egg, 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. 1 



Van Beneden describes 

 the astral rays, both in 

 Ascaris 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- pig ^_ SY}ght]y schema tic 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. 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 rays are attached. 



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- 



