THE ANIMAL CELL 23 



a distinct 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 the irregular 

 breaking up of the protoplasm into numerous masses, each 

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

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

 appears on the cell, develops into bodies 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. Direct cell division (Amce&a). A, active specimen with pseudopodia; B, becoming 

 spherical preliminary to division; C, beginning of elongation and constriction; D, later stage; JB, 

 daughter cells forming pseudopodia. ec. clear ectoplasm; en, granular endoplasm;/, food vacuole; 

 , nucleus; ps t pseudopodium; v. pulsating vacuole. 



Questions on the figure. Why is this properly called direct division? What 

 structures are divided? Are the resulting halves exactly or merely roughly equal, 

 apparently? Do you see any possible gain to the organism in such a division as 

 this? 



