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 



D 



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 j/zW;^^, nucleolus still 

 present; above, 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 type alone). F. The mitotic figure established; 

 e.p. the equatorial plate of chromosomes. (^Cf. Figs. 21, 27, 32, etc.) 



