694 TEXTBOOK OF ZOOLOGY 



the formation of a characteristic organ for that time and place in 

 the development of one of the more complex organisms. 



Dominant areas sometimes produce substances which diffuse to sur- 

 rounding cells and determine their subsequent proliferation and dif- 

 ferentiation. Such dominant areas are called organizers. The dorsal 

 lip of the blastopore of the amphibian eg^, which determines the 

 formation of the central nervous system, is a classic example. Cells 

 from the vegetal pole of the amphibian egg will form nervous tissue 

 under the influence of the organizer if such cells are placed over 

 it instead of the cells normally occurring there. Organizers are 

 formed in the region of every regenerating structure, if not pre- 

 viously present. Thus, the stump of the leg of a salamander induces 

 the cells of the blastema to form those structures which are lacking. 

 In some cases, as in hydroids, if the dominant area itself is injured 

 organs characteristic of that region are regenerated rather than the 

 portion removed. 



In the complex organisms, there finally results a maze of inter- 

 dependent and integrated, locally controlled regions under the gen- 

 eral dominance of the body as a whole. Each organ has a character- 

 istic size and rate of activity in proportion to the other parts. These 

 relations may be maintained from a food-supply in the blood stream 

 kept at a level which is relatively uniform for all portions of the 

 body, through different rates of absorption and utilization of food by 

 the different kinds of cells. 



As development proceeds, an ever greater proportion of cells be- 

 comes functionally differentiated, and the rate of proliferation de- 

 creases correspondingly. Some cells, such as the germ cells and the 

 cells which replenish the supply of blood cells, maintain their physio- 

 logical youthfulness and rate of proliferation, while others, such as 

 nerve cells, become irreversibly differentiated and cease proliferation 

 altogether. Animals cease to grow when a balance is reached between 

 normal wearing out of tissue cells, on the one hand, and proliferative 

 activity on the other. 



Experiments with tissue cultures have clearly demonstrated that 

 animal cells are potentially immortal. Removed from the dominance 

 of the organ and body of which they form a part and given adequate, 

 sanitary, nutrient conditions, they multiply indefinitely, forming 

 other cells like themselves. Tissue cells from vertebrate animals do 

 not organize new animals in tissue culture. Apparently, vertebrate 



