REGENERATION AND DEATH 195 



How has nature proceeded in order to produce a 

 higher type of animal, one in which the number of cells 

 is much greater? Very ingeniously. She provides 

 the developing organism with a food supply which 

 it carries itself. If, for instance, you recall the egg of 

 the salamander, which I showed you upon the screen, 

 you will remember that that is a structure of consider- 

 able size, and its size is due to the accumulation of 

 food material, material which we designate by the 

 term yolk granules, which lie in the living protoplasm 

 of that germ. This supply of food is so great that it 

 will last the organism a considerable period. While 

 it is growing it has nothing to do but to digest that 

 food supply which it already possesses. It does not 

 have to exert itself to obtain it, and no further diges- 

 tive process is necessary than that inherent in all living 

 protoplasm. So the young salamanders develop under 

 most advantageous conditions, and can actually pro- 

 duce a much greater number of cells because it is 

 possible, with this internal food supply, for the growth 

 to go on only with the cells of the embryonic or 

 youthful type for a considerable period, and then, 

 when their number has considerably increased, steps 

 in the process of differentiation. 



In the higher animals the accumulation of food for 

 the nourishment of the germ is carried yet further. 

 As you know, the egg of the bird is much bigger than 

 that of the salamander. Again, in the highest ani- 

 mals, in the placental mammals, there are other special 

 contrivances which nature has introduced to secure 



