THE ANIMAL CELL 



23 



tinct motion of their own. The muscle cells of higher animals 

 possess the power of contraction and motion in a high degree. 



40. Reproduction. — The cell grows as a result of the nu- 

 tritive processes and reaches the limits of size determined by 

 its special conditions. The internal and external conditions 

 together constitute a stimulus to the breaking up or division of 

 the protoplasmic unit. This may occur (i) by spore forma- 

 tion, breaking up of the protoplasm into numerous masses, each 

 of which has the essential qualities of the whole (Fig. 6, A, and 

 B) ; (2) by budding, in which a process, or several processes, 

 appears on the cell, develops into a body like the original cell, 

 and finally becomes separate from it (Fig. 6, D) ; (3) by division. 

 in which there is a division of the original protoplasm into 

 two essentially equal parts. In the last case neither of the 

 cells can be considered the parent of the other. 



41. Cell Division. — Cell division may be effected in either 

 of two ways, (a) by direct or amitotic division, in which the 



Fig. 7 



B 



C 



D 



Fig.' 7. Direct nuclear division (amitosis), in the egg-follicle of a cricket. — (Reproduced by 

 courtesy of The Macmillan Company Jrvm Dahlgren b' Kepner's ** Textbook of the Principles of 

 Animal Histology,") 



Questions on the figure. — Why is this appropriately called direct division? 

 How does it compare with the process pictured in Fig. 8, in method and in results? 



nucleus and cell merely constrict into two nearly equal parts 

 without any elaborate preliminary rearrangement of the nuclear 

 material (Fig. 7) ; and (6) indirect or mitotic division. The latter 

 is the usual method and is very. complicated. By means of it 

 a very even division of the substances and structures of the 

 nucleus, especially, seems to be secured. 



