88 



MANUAL OF PHYSIOLOGY. 



specific inherited characteristics no doubt, but to all appearances 

 the same as the motile sentient growing protoplasm of an indif- 

 ferent cell. By a process of fission or budding constantly going 

 on in this deepest layer of these cells, new protoplasmic units are 

 produced. These become distinct individuals, and occupy the 

 position of the parent cell, which, having produced offspring, is 

 moved one place nearer the surface, away from the supply of food. 

 The new cell in time gives rise to offspring, and having attained 

 reproductive maturity, is in turn moved onward toward the sur- 

 face. The result of this is that its supply of nutrition diminishes, 



FIG. 89. b 



Division of egg cell. (Gegenbaur.) 



the evidences of reproductive activity disappear, and at a certain 

 point all signs of protoplasmic life are lost. But on its way from 

 the seat of its origin to the surface, it makes use of its limited 

 supply of nutrition for the purpose of manufacturing a special 

 kind of material which, if present at all, only occurs in the 

 minutest traces in ordinary protoplasm. As the cell moves toward 

 the surface, it loses its protoplasmic characters, becomes tougher 

 and drier, and finally nothing but the special horny material 

 remains. Thus from the birth of the cell, its energies are devoted, 

 first to its own growth, then to the reproduction of its like, and 



