REPRODUCTION 7 



As we can follow step by step the gradual evolution of 

 the higher types of organisms from the lower by the ag- 

 glomeration and gradual differentiation of cells, so we can 

 trace a parallel development in each single individual. 

 Every animal or plant starts life as a single cell. This 

 mother-cell divides and redivides, forming a mass of cells, 

 which gradually differentiate into layers, tissues, and organs, 

 finally to build up the complete body in all its complexity. 

 The construction of the body, then, from the first mother- 

 cell to its final size and form, is thus due to a continuous 

 reproduction of cells. The growth of each part of the body 

 is kept up by the multiplication and addition of cells arising 

 from the old cells already present. In fact, growth, as we 

 see, is nothing but a process of reproduction of cells. As 

 long as the organism grows, the material for the additional 

 growth is supplied by the old cells, which divide to form 

 the new ones. 



But, at the same time, with this process of growth and 

 building up of the body another process is going on in the 

 organism — a gradual wearing down and simultaneous re- 

 newal of the worn-out parts of the body. Cells grow old, 

 die, and are replaced by new ones. This process is a very 

 familiar one. Everybody knows that skin, hair, and nails 

 are periodically renewed under normal conditions. The 

 small scales of the skin which are regularly thrown off are 

 nothing else than old dried-up cells, which, if in excess, 

 form on the head the well-known dandruff. Hair, which 

 is nothing but a cylinder of ceUs, falls out regularly, and is 

 replaced. The moulting of birds, snakes, crabs, etc., is a 

 well-known phenomenon. There is every reason for as- 

 suming that the other cells in the different organs of the 

 body are also periodically renewed, as we know that the 

 mucous linings of mouth, intestine, etc., shed their cells 

 regularly. 



We recognize, then, a constant process of regeneration 

 of the body-cells as a normal physiological function. If 

 the gradual decay of cells is not sufficiently made up by 



