CHARACTERISTICS OF PROTOPLASM 7 



formation of various more complex chemical substances on the other, some of 

 which may be stored up in the cell for future use, and are called secretions, 

 and others, like carbon dioxide, for example, and bodies containing nitrogen, 

 are eliminated as excretions. 



The Power of Growth. In protoplasm it is seen that the two processes of 

 waste and repair go on side by side, and so long as they are equal the size 

 of the animal remains stationary. If, however, the building up exceed the 

 waste, then the animal grows; if the waste exceed the repair, the animal 



FIG. 7. Diagram of an Ovum (a) Undergoing Segmentation. In (6) it has divided into two. 

 in (c) into four; and in (d) the process has ended in the production of the so-called "mulberry mass." 

 (Frey.) 



decays; and if decay go on beyond a certain point, life becomes impossible, 

 and the animal dies. 



The power of increasing in size, although essential to our idea of life, is not, 

 it must be recollected, confined to living beings. A crystal of common salt, 

 for example, if placed under appropriate conditions for obtaining fresh mate- 

 rial, will increase in size in a fashion as definitely characteristic and as easily 

 to be foretold as that of a living creature; but the growth of a crystal takes 

 place merely by additions to its outside; the new matter is laid on particle by 

 particle, and layer by layer, and, when once laid on, it remains unchanged. In 

 a living structure, where growth occurs, it is by addition of new matter, not 

 to the surface only, but throughout every part of the mass, and this matter be- 

 comes an intimate part of the living substance. 



The Power of Reproduction. The ameba, to return to our former illus- 

 tration, when the growth of its protoplasm has reached a certain point, mani- 

 fests the power of reproduction, by splitting up into (or in some other way pro- 

 ducing) two or more parts, each of which is capable of independent existence. 

 The new amebae manifest the same properties as the parent, perform the same 

 functions, grow and reproduce in their turn. This cycle of life is being con- 

 tinually passed through. 



In more complicated structures than the ameba, the life of individual 

 protoplasmic cells is probably very short in comparison with that of the organ- 

 ism they compose ; and their constant decay and death necessitate constant re- 

 production. The mode in which this takes place has long been the subject of 

 controversy. 



It is now very generally believed that every cell is descended from some 

 pre-existing mother cell. This derivation of cells from cells takes place by 

 gemmation, which essentially consists in the budding off and separation of 

 a portion of the parent cell; or by fission or division. 



