66 



CELL-DIVISION 



convolution becomes more open ('* open spireme"). In some cases 

 there is but a single continuous thread ; in others, the thread is from 





E 



Fig. 25. — Diagrams showing the prophases of mitosis. 



A. Resting cell with reticular nucleus and true nucleolus; at c the attraction-sphere containing 

 two centrosomes. B. Early prophase ; the chromatin forming a continuous spireme, nucleolus still 

 present : abo%-e, the amphiaster {a). C. D. Two different types of later prophases. C. Disappear- 

 ance of the primary spindle, divergence of the centrosomes to opposite poles of the nucleus (exam- 

 ples, some plant-cells, cleavage-stages of many eggs). D. Persistence of the primary spindle (to 

 form in some cases the " central spindle"), fading of the nuclear membrane, ingrowth of the astral 

 rays, segmentation of the spireme-thread to form the chromosomes (examples, epidermal cells of 

 salamander, formation of the polar bodies). E. Later prophase of type C\ fading of the nuclear 

 membrane at the poles, formation of a new spindle inside the nucleus; precocious splitting of the 

 chromosomes (the latter not characteristic of this tvpe alone). E. The mitotic figure established; 

 e.f. the equatorial plate of chromosomes. {€/. Figs. 21, 27, 32, etc.) 



