INTRODUCTION 



II 



Ninety-seven per cent of protoplasm consists of the following 

 four elements: - 



Oxygen 65.0 per cent 



Carbon 18.5 per cent 



Hydrogen n.o per cent 



Nitrogen 2.5 per cent 



These and other elements form rather definite compounds. 

 The principal inorganic constituents of protoplasm are (i) water, 



which comprises more than 50 per .^____ 



cent of the weight of most animals, 

 (2) salts, such as the chlorides, car- 

 bonates, and phosphates, and (3) gases, 

 such as oxygen and carbon dioxide. 



The organic compounds found in pro- 

 toplasm comprise the proteids, carbo- 

 hydrates, and fats. Proteids consist of 

 large molecules which always contain 

 carbon, oxygen, hydrogen, and nitro- 

 gen. They do not dissolve in water, 

 but absorb quantities of this fluid, 

 swelling up like a sponge. Other 

 peculiarities are their inability to pass 

 through animal membranes and their 

 property of coagulation or clotting. 

 Carbohydrates are compounds of car- 

 bon, hydrogen, and oxygen, the last two 



nearly always occurring in the same 



..... . . FIG. i. Alveolar struc- 



proportion in which they are found in t ure of the protoplasm of an 



water (H 2 0). Starches and sugars are epidermis cell of an earth- 



. worm. (From Verworn, after 



common carbohydrates. Some living Butschli.) 



substances apparently do not contain 



this compound. Fats are likewise not invariable constituents of 



protoplasm. The protoplasm of each species of animal differs from 



that of every other species, but in all it has similar characteristics. 



