2i8 Education through Nature 



with the condition, such as age, etc., of each cell. The 

 power of the cell to produce energy and to assimilate 

 food, restoring the waste, is gradually lost by the cell, 

 and this law, from which there seems to be no appeal, 

 is responsible for the old age and final death of all 

 living things. 



Secretion. Besides the energy given off in the form 

 of motion and heat, substances are produced by various 

 cells in the body. Such production involves work 

 and consequently waste and repair. Substances pro- 

 duced either by the metabolic activity of the cells, 

 or by filtration of fluids from the blood, that are useful 

 in the body, and apparently intended for use, as in 

 the case of digestive fluids, are called secretions. 

 Digestive secretions of some kind probably exist in 

 in all animals. But there are many other substances 

 that may properly be considered as secretions. Thus 

 the intercellular substance in cartilage and bone is 

 probably a secretion; so, too, the plasma of the blood 

 is originally a secretion. Many secretions harden 

 into protective structures, as the exoskeleton of insects 

 and Crustacea and the shells of mollusks. Even the 

 cell-membrane, where such exists, as in the egg, is to 

 be regarded as partly or wholly a secretion; and we 

 have already seen that the white of the bird's egg, 

 and the gelatinous covering of the amphibian egg, are 

 secretions of the oviduct. The yolk of birds' eggs is 

 probably partly a secretion of the follicle-cells, partly 

 an internal secretion of the egg protoplasm, the egg- 

 shell being a hardened secretion of the oviduct. 



Excretion. All activity involves waste. Waste re- 

 sults from the breaking down of complex organic com- 

 pounds into simple inorganic compounds. Such 

 simple inorganic compounds have no energy that can 

 be used in the body, and consequently are useless so 

 far as maintaining the body is concerned. Hence, 

 if allowed to accumulate, they interfere with the normal 



