THE DIVISION OF THE PROTOPLAST 



61 



nema which again becomes more closely coiled; thus the chromosome soon 

 becomes a thicker, smoother, double body comprising two chromatids, each 

 composed of chromonema and matrix. In ordinary preparations the 

 deeply stained matrix renders the chromonema invisible, but suitable 

 methods reveal it. In some cases there is visible evidence that the 

 chromonema in each chromatid is in turn longitudinally double, the whole 

 chromosome by the end of the prophase therefore having four half- 

 chromatich. The nucleolus commonly disappears late in the prophase as 

 the matrix becomes abundant and stainable. 



The nucleus next passes rapidly through a stage known as the pro- 

 vietaphasc into the mctaphasc. This involves a complicated series of 

 changes in which the karyolymph, probably with the cooperation of some 

 cytoplasmic substance, liecomcs transformed into the achromaiic figure, or 





I 





_f3 



Fiu. 41. — Stages in mitosis in root tips, a, anaphase; b, telophase; c, (/, early prophase; 

 e, late prophase. {After L. W. Sharp.) 



spindle. That this change consists primarily in a definite rearrangement 

 of materials, presumably protein chains, into positions parallel with the 

 longitudinal axis of the spindle, and a differentiation into two components, 

 one relatively firm and the other more fluid, is indicated by several lines 

 of evidence: the spindle, unlike the material previously present, is aniso- 

 tropic; it offers axial resistance to swelling or shrinking agents; it splits 

 longitudinally in shrunken cells; Brownian movement of occasional parti- 

 cles in the more fluid regions is greatest parallel to the longitudinal axis ; 

 fixation usually gives the spindle a longitudinally striated or fibrillar 

 aspect. 



The spindle in root tips commonly begins its development at two 

 opposed poles of the nucleus, apparently outside the nuclear membrane 

 shrinking inward in these regions (Fig. 42). Sometimes it develops more 

 or less simultaneously throughout the nucleus with no membrane shrink- 

 age. In either case the membrane (eventually disappears, leaving the 

 chromosomes, which have meanwhile moved toward the equatorial plane 

 of the nucleus, in the midst of the spindle. The double chromosomes, 



