THE STUDY OF ZOOLOGY 7 



From the purely physical side, protoplasm resembles sub- 

 stances like glue or gelatin rather than crystalline substances 

 like cane sugar or sodium chloride. When living cells are sub- 

 jected to dissection under the microscope with extremely fine 

 needles, the more solid portions (gels) can be drawn out into 

 thin threads having considerable tenacity. The viscous prop- 

 erties of protoplasm can also be demonstrated by centrifuging 

 living cells. Under the pull of the centrifugal force the heavier 

 components are dragged through the cell and become arranged in 

 layers according to density. If the centrifugal force is great 

 enough, the cells can be pulled into dumbbell shapes, the proto- 

 plasm between the heavier and lighter half being drawn out into 

 a thin strand, which may finally snap. 



The mere touch of a needle to a living cell may cause liquefac- 

 tion or the reverse change, gelation, to occur with extreme 

 rapidity. The reversibility from solid to liquid, or liquid to 

 solid, is one of the striking features of protoplasm and one which 

 may be regarded as a characteristic of the living colloid. 



Composition of Protoplasm. — Protoplasm is not a definite 

 chemical compound in the technical sense. It is rather a chem- 

 ical complex of substances, the percentage composition of which 

 is not the same for all kinds of protoplasm. The same kinds of 

 substances occur in all forms of protoplasm, but the proportions 

 and the actual composition of the different constituents vary. 

 The most abundant constituent is water, which accounts for 

 from about 75 per cent to more than 90 per cent of the total 

 weight of the protoplasm. Water is utilized as a solvent and as 

 such plays an important part in bringing about chemical reac- 

 tions. It is also required to maintain the colloidal state. In 

 some marine forms as in jellyfishes, water forms 99.8 per cent of 

 the body weight. The human body contains about 65 per cent of 

 water, the weight of the bones lowering the average. Inorganic 

 salts such as sulphates, chlorides, phosphates, and carbonates of 

 sodium, potassium, magnesium, and calcium, and small amounts 

 of iron make up about 5 per cent of the whole. Small amounts 

 of iodine, manganese, copper, zinc, barium, and silicon are present 

 in varying amounts. Of the remaining constituents, viz., pro- 

 teins, carbohydrates, fats and extractives, which are organic in 

 nature, protein seems to be the most important and forms about 

 15 per cent of the human body. Protein is a very complex 



