THE MITOTIC CYCLE 



causes the contents to re-enter the cell once again. Sometimes when an 

 appreciable proportion of the endoplasm is being extruded into a 

 large bubble, the contents of one will be sucked out by the formation of 

 another; the older bubble will then shrink and collapse and lose its 

 rounded outline during the return stroke. Before any further analysis 

 of the phenomenon could be attempted it would be necessary to know 

 something of the hydrostatic pressure within the cell at the time of 

 cleavage. 



Cytokinesis in plants 



In plants, as in animals, there is no uniform pattern of cell division. 

 A classification of the various methods in plants by which the daughter 

 cells -are separated is given by Yamaha. ^^^ The most usual course of 



10 20 30 VO 50 



70 80 90 100 110 120 130 1¥0 150 160 170 180 

 Time sec 



Figure 56 'Bubbling' in a chick cell during anaphase and cleavage. Dimen- 

 sions of bubbles measured from frames of a film record. The four outlines 

 of the cell refer to the times indicated. 



events is that a cell plate is formed midway between the daughter 

 nuclei; this is formed by the coalescence of a row of granules which are 

 either formed, or take up their position in this plane. In the flowering 

 plants, it seems that it is not the anaphase spindle which forms the cell 

 plate, but another orientated body which succeeds it; to this the name 

 'phragmoplast' is given. According to Becker^^^ ^^^ there is some 

 variation in the degree to which this body is distinct from the anaphase 

 spindle. These events in the living Tradescantia staminal hair cell (Plate 

 V (9 p-r) ) are described by Barber :^^^ 



During metaphase and anaphase it is extremely difficult to make out any definite 

 spindle in the living cell. Whilst the chromosomes are contracting to form the telo- 

 phase nuclei the whole cell undergoes very vigorous cytoplasmic movements. 



146 



