14 



COLLEGE ZOOLOGY 



from its important relation to cell division, to be described later, 

 seems to be the control of the activities by which the protoplasm 

 is elaborated. 



The cytoplasm, from its direct relation to the outside world, 

 is the seat of such functions as irritability, absorption, digestion, 

 excretion, and respiration. The centrosome is of importance 

 during cell division. The cell covering may serve for pro- 

 tection or support, or may be extremely delicate and have sig- 

 nificance only as it helps to control the absorption of certain 

 fluids. Plastids may represent stored food or waste products; 

 some of them, however, have other functions, e.g. the chloro- 

 plasts, which carry on photosynthesis in many plants and a*few 

 animal cells. 



(3) Cell Division. — Cells multiply either by direct division 

 (amitosis) or indirect division (mitosis). In amitosis (Fig. 11) 



:■ ' -' ; - - '■■ 



■•■■ : j^0^K : \ 



■^-"K-Vf-X 



■ 



Fig. 3. — Amitosis. Amitotic nuclear division in the follicle cells of a 

 crickefs egg. (From Dahlgren and Kepner.) 



the nucleus is either pinched in two in the middle, or a 

 plate is formed in the plane of division, which later be- 

 comes double, and then the two plates separate, or two nuclear 

 membranes are built up inside of the old membrane. The cell 

 body then divides, though in many cases this process does not 

 occur (Fig. 3). Amitosis is characteristic of senescent cells. 



Mitosis is the usual method of nuclear division. It consists 

 of a series of complex processes that may be arranged into 

 four phases. Constant reference to Figure 4 will make clear the 

 following brief account. 



(a) During the prophase the chromatin granules that are scat- 

 tered through the nucleus in the resting cell (A) become ar- 



