. The cell in development and inheritance. Cells; Cells. THE MECHANISM OF MITOSIS 71 ac evidence derived from the study of the Ascaris &g^, 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 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 sp
. The cell in development and inheritance. Cells; Cells. THE MECHANISM OF MITOSIS 71 ac evidence derived from the study of the Ascaris &g^, 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 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 K -' . ,'<\f^ "^T^''"'- •. 1 1<^-<S the cell-periphery, the base of the cone forming the "polar ; These rays, opposing the action of the principal cones, not only hold the centrosomes in place, but, by their con- ^.^ 34—Slightly schematic figures of dividing eggs tractions, drag them apart, of Ascaris, illustrating Van Beneden's theory of mitosis. and thus cause an actual [Van beneden and julin.] A. Early anaphase; each chromosome has divided. into two. B. Later anaphase during divergence of the daughter-chromosomes, Antipodal cone of astral rays; cortical zone of the attraction-sphere; i. in- terzonal fibres stretching between the daughter-chromo- somes ; medullary zone of the attraction-sphere; principal cone, forming one-half of the contractile spindle (the action of these fibres is reinforced by that of the antipodal cone) ; sub-equatorial circle, to which the astral rays are attached. divergence of the centres. The remaining astral rays are attached to the cell- periphery and limited by a sub-equatorial circle. Later observations indi- cate, however, that this arrangement of the astral rays is not of general occurrence
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectcells, bookyear1896
