22 



ZOOLOGY. 



blood cells, sexual cells, etc., which have a distinct motion of 

 their own. The muscle cells of higher animals possess the 

 power of contraction and motion in a high degree. 



39. 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 

 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. 5, A and 

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

 appear on the cell, develop into bodies like the original cell, and 

 finally become separate from it (Fig. 5, D) ; (3) by division, 

 in which there is a division of the original protoplasm into 

 two essentially equal parts. In this case neither of the cells 

 can be considered the parent of the other. 



40. Cell Division. Cell division may be effected in either 

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



FIG. 6. 

 A 



FIG. 6. Direct cell division (Amoeba). A, active specimen with pseudopodia; B, be- 

 coming spherical preliminary to 'division; C, beginning of elongation and constriction; 

 D, later stage; E, daughter cells forming pseudopodia. ec, clear ectoplasm; en, granu- 

 lar endoplasm; f, food vacuole; n, nucleus; ps, 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? 



