68 



OENEllAL PRINCIPLES OF ZOOLOGY. 



cell division which controls both the division of the nucleus and 

 that of the cell itself. 



Multiplication of Cells. — Increase in cells occurs exclusively 

 by division or by budding (gemmation). Most common is binary 

 division in which a circular furrow appears on the surface of the 

 cell, deepens and cuts the cell into two equal parts. Multijile 

 division is more rare and can only occur in multinucleate cells. 

 Here the cell divides simultaneously into as many (sometimes 



hundreds) daughter-cells as 

 there were nuclei present. In 

 all forms of division the simi- 

 larity of the products is char- 

 acteristic, while in budding the 

 resulting j^arts are unequal. 

 In budding one or more 

 , smaller daughter-cells, the 

 ,^ buds, are constructed from a 

 large mother-cell (fig. 20). 



Direct Cell Division 



Every cell division is accom- 

 panied by nuclear division or 

 Fig. 20.-Ceii bnading. Podophrya gemmi- ^t least presumes that nuclear 



paro iTith buds (a) which separate and form rlivToinn lias nravmnsK- nn 



free young ()j). iv, nucleus. uuisiou nab prcMousi} Oc- 



curred. Direct and indirect 

 division are recognized. Direct division is most common in 

 Protozoa, and especially in nuclei with abundant chromatin (fig. 

 20, 145). The nucleus is elongated and is divided by constriction, 

 in the same way that the cell itself constricts. Since the proto- 

 plasm has no special arrangement with regard to the dividing 

 nucleus (the latter besides protected by its membrane), we must 

 conclude that the nucleus divides itself and is not passively 

 divided. The dividing force resides in the achromatic framework, 

 which correspondingly often exhibits a certain arrangement, a 

 fibrous structure in the direction of the elongating nucleus. 



Indirect Cell Division, Karyokinesis.— Indirect cell division, 

 karyokinesis or mitosis, is most beautifully shown in cells, poor in 

 chromatin, which possess a centrosome. The process is introduced 

 by a division of the centrosome (fig. 21). The daughter centro- 

 somes migrate to two opposite poles of the nucleus, which now 

 loses its membrane and becomes the nuclear spindle. The 

 characteristics of the spindle are that it is drawn out into points 

 at two poles which are indicated by the position of the centre- 



