24 NUCLEAR DIVISION 



stage by a series of changes broadly the reverse of those taking 

 place at the commencement of nuclear division. That is, the 

 individuality of the chromosomes becomes obscured, owing to 

 the formation of numerous cross-connections between them, and 

 through the separation of the two halves of each chromosome, 

 which only remain joined at the ends and by fine processes in 

 between. With the appearance of nucleoli and a new nuclear 

 membrane, the resting nucleus with its chromatin-reticulum is 

 again established. 



From the original nucleus two daughter-nuclei are hence 

 formed, either of which contains a half of each of the chromo- 

 somes of the parent-nucleus. As the process is repeated at every 

 division, all the vegetative cells 1 of the plant come to possess 

 nuclei with the same number of chromatin-masses. 



In addition to the contractile fibres which were attached 

 to the chromosomes, other fibres extend through the cytoplasm 

 from pole to pole. These persist for some little time after the 

 establishment of the daughter-nuclei, and develop thickenings 

 in the equatorial plane, an appearance probably due to a modi- 

 fication of the cytoplasm in this region. These thickenings be- 

 come more conspicuous as they extend horizontally across the 

 parent-cell, till finally they join to form a complete diaphragm 

 of modified cytoplasm, the cell-plate (Fig. 12, B). Within the 

 latter there is formed a thin membrane of cell-wall substance 

 which separates the two daughter-cells, and is traversed by the 

 fine cytoplasmic connections representing the fibres. The latter 

 persist, even in the adult condition, so that the protoplasts of 

 adjacent cells of most plants are connected by living matter. 

 Subsequently further layers, consisting largely of cellulose, are 

 deposited on either side of the original membrane, which is the 

 only part of this separating wall common to the two daughter- 

 cells. These additional layers frequently become chemically 

 changed in various ways, but the original separating wall, 

 termed the middle lamella (Figs. 14, 15, m.L), does not usually 

 undergo such profound modification, so that it is readily dis- 

 tinguished by its different density, and often stains more deeply. 2 



1 For details of nuclear division in reproductive cells, see p.p. 305, 306, 



2 The separating wall in many Algae arises as an ingrowing diaphragm 

 (Fig. 10 |, E, p. 196), and is not always directly related to nuclear division. 



