4 8 



CELL-DIVISION 



Every species of plant or animal Jias a fixed and cJiaracteristic num- 

 ber of chromosomes^ wJiich regularly recurs in tJie division of all of its 

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



D 



E 



F 



Fig. 19. Diagrams showing the prophases of mitosis. 



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

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

 still present; above, the amphiaster (a). C. D. Two different types of later prophases; C. Dis- 

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

 (examples, many plant-cells, cleavage-stages of many eggs). /.>. 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 spirenne-thread to form the chromosomes (examples, epi- 

 dermal cells of salamander, formation of the polar bodies). /;. 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). /'. The mitotic 

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



