308 



ON THE INTERNAL FORM 



[CH. 



chromosomes into their new places* (Figs. 92, 93); and its chromo- 

 somes form once more an alveolar reticulum and may occasionally 

 form another spireme at this stage. A boundary-surface, or at least a 

 recognisable phase-difference, now develops round each reconstructed 

 nuclear mass, and the spindle disappears (Fig. 93 b). The centrosome 

 remains, as a rule, outside the nucleus. 



7. On the central spindle, in the position of the equatorial plate, 

 a "cell-plate," consisting of deeply staining thickenings, has made 

 its appearance during the migration of the chromosomes. This cell- 

 plate is more conspicuous in plant-cells. 



otfrottron spncft 



,d>iopptQr,ng spmifte 



Htcon^rucffd dough/trnuei^t 



Fig. 93 B. 



8. Meanwhile a constriction has appeared in the cytoplasm, and 

 the cell divides through the equatorial plane. In plant-cells the 

 line of this division is foreshadowed by the "cell-plate," which 

 extends from the spindle across the entire cell, and spHts into two 

 layers, between which appears the membrane by which the daughter- 

 cells are cleft asunder. In animal cells the cell-plate does not attain 

 such dimensions, and no cell-wall is formed. 



The whole process takes from half-an-hour to an hour; and this 

 extreme slowness is not. the least remarkable part of the pheno- 

 menon, from a physical point of view. The two halves of the 



* The spindle has no actual threads or fibres, for Robert Chambers's micro- 

 needles pass freely through it without disturbing the chromosomes: nor is it 

 visible at all in living cells in vitro. It seems to be due to partial gelation of the 

 cytoplasm, under conditions which, whether they be mechanical or chemical, are 

 not easy to understand. 



