48 CELL-DIVISION 



Every species of plant or animal has a fixed and characteristic num- 

 ber of chromosomes, zvJiich regularly recurs in the division of all of its 

 cells; and in all forms arising by sexual reproduction the number is 



D 



Fig. 19. —Diagrams showing the prophases of mitosis. 

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

 in'^ two centrosomes. B. Earlv prophase ; the chromatin forming a continuous spireme, nucleolus 

 sti'll present- above, the amphh.ster {a). C. D. 'IVo different types of later prophases; C. Dis- 

 appearance 'of the primary spindle, divergence of the centrosomes to opposite poles of the nucleus 

 (examples manv plant-cells, cleavage-stages of many eggs). /.). Persistence of the' primary 

 spindle (to form' in some cases the "cent.al spindle"), fading of the nuclear membrane, ingrowth 

 of the astral ravs segmentation of the spireree-thread to form the chromosomes (examples, epi- 

 d-rmal cells of salamander, formation of the polar bodies). E. Later prophase of tvpe C ; fadmg 

 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). I. The mitotic 

 figure established ; e.p. The equatorial plate of chromosomes. (Cf. Figs. 16. 21, 24.) 



