28 CELL-CONTENTS AND CELL-WALLS. 



carefully selected, fixed, and stained. However, there are 

 a few cases in which it is possible to trace some of the 

 stages of nuclear division (mitosis, or karyokinesis) and 

 cell-division without the use of fixative or stains. 



(a) The processes of division can be, in part at any rate, observed 

 in living cells. Carefully open a flower-bud of Tradescantia which 

 is almost ready to open (choose a warm day or use a plant that has 

 been kept for some time in a warm place, so that growth has been 

 vigorous), remove the stamens, mount them in 2 per cent, sugar 

 solution, cut off the anthers, cover the filaments, and examine the 

 hairs on the latter with high power. Each hair consists of a row 

 of cells, having relatively large nuclei. In most of the cells the 

 nucleus will appear rounded and definite in form (resting nucleus), 

 but in the longer cells at or near the end of the hair the nucleus 

 has an elongated form and ill-defined appearance (dividing nucleus). 



In a resting nucleus note (1) the fine chromatin threads 

 forming a network and giving the nucleus a granular appearance, 

 (2) the highly refractive nucleoli usually one or two in number, 

 sometimes more. 



In the dividing 1 nuclei the following stages can be made out: 

 (l)the nucleus grows larger ; (2) the threads become thicker ; (3) the 

 network breaks up into a number of rod-like chromosomes, at first 

 curved ; (4) the dividing nucleus becomes spindle-shaped, with the 

 chromosomes straightened and arranged in two groups, one group 

 on either side of the equator of the spindle (each original chromo- 

 some has split longitudinally into two, one half passing to one side 

 of the equator and the other half to the other side, but this is not 

 easily observed), the cell meanwhile having grown in length ; (5) the 

 chromosomes of each group become curved again, and join up to 

 form the chromatin network of the new nucleus ; (6) the cell-plate 

 is formed at the equator of the spindle, by the fusion of granules 

 which have appeared here ; (7) the spindle widens out, so that the 

 cell-plate reaches the outer wall of the cell, which is thus divided 

 into two cells by the new cell-wall. 



(6) Some hairs of Tradescantia should be stained, in order to 

 bring out the details clearly. For this purpose we may either use 

 a single stain, or two stains of which one will show up the chro- 

 matin and the other the fine spindle-threads. Of single stains, 

 methyl green or haematoxylin should be used. For double staining 

 use first safranin and then gentian violet ; the former stains the 

 chromosomes and nucleoli, the latter the threads of the spindle 

 connecting the two new nuclei. In the case of living cells it is 

 better to use a single stain, and methyl green answers well with 

 the hairs of Tradescantia. 



(c) In order to study both nuclear division and the changes 

 undergone by young cells, root-tips afford good material. The 



