54 



THE CELL 



of cell-division is uncommon in higher animals where Flemming con- 

 siders it a degenerative phenomenon rather than a normal method 



of cell-increase. It is a 

 common method of cell- 

 division in the protozoa. 

 Indirect Cell-divi- 

 sion — Mitosis (Figs, g, 

 lo). — In this form of cell- 

 division also, the nucleus 

 divides into two daughter 

 nuclei, and the cell into 

 two daughter cells, but 

 only after they have 

 passed through certain 

 characteristic and compli- 

 cated changes. These 

 changes occur as a contin- 

 uous process, but it is con- 

 venient for clearness of 

 description to arbitrarily 

 divide them into stages or 

 phases. Thus we recognize in mitosis: (a) the prophase; (b) the 

 metaphase; (c) the anaphase; (d) the telophase. The prophase is 

 the stage of preparation on the part of the nucleus for division; 

 the metaphase, the actual separation of the nuclear elements; the 

 anaphase, the formation of the two daughter nuclei; the telophase, 

 the reconstruction of the two daughter resting nuclei and the divi- 

 sion of the cytoplasm. 



{a) The Prophase (Fig. 9, B, C, D) is marked by the following 

 changes : 



I. The centrosome, if single, divides into two daughter centro- 

 somes. In most actively dividing cells, however, the centrosome 

 is at this stage already double (Fig. 9, A) having divided as early, 

 frequently, as the anaphase of the preceding mitosis. 



The two daughter centrosomes, each surrounded by its attraction 

 sphere, now move apart but remain connected by fibrils, probably 

 derived from the linin (Fig. 9, B). These fibrils form the ce«/ra7 or 

 achromatic spindle. Two other sets of fibrils radiate from each cen- 

 trosome — one, known as the polar rays, passes out toward the periph- 

 ery of the cell; the other, known as the mantle fibres, extends from the 





^ -A 

 " \ 

 \ 

 \ 



jzr 



Fig. 8. — Epithelial cell from bladder showing 

 amitotic division of its nucleus. (Nemileff.) /, 

 Cytoplasm; //, two daughter nuclei; ///, fibrils 

 uniting daughter nuclei. 



\ 



