Protoplasm. 33 



involves the expenditure of a certain amount of energy. 

 Protoplasm, in virtue of its being composed of a number of 

 very complex and unstable compounds, constitutes a large 

 store of potential energy. In consequence of the trans- 

 formation of part of this rxrtential energy into kinetic energy 

 the various functions of protoplasm come to be performed ; 

 in other words, the- "various functions of protoplasm are 

 manifestations of kinetic energy. In the expenditure of 

 energy the complex compounds become broken down into 

 simpler compounds and often into elements ; hence the 

 presence of so many transitional substances and simple 

 compounds in the chemical composition of proto^asm. 

 We have now to consider what these functions are, and for 

 that purpose we will consider a naked cell, such as that re- 

 presented at fig. 3. 



We note first that it possesses contractility that is to 

 say, that it is capable of motion as a whole or in certain 

 parts. Protoplasmic motion may be definite or indefinite. 

 Indefinite motion consists in the protrusion and withdrawal 

 of finger -like prolongations of protoplasm from any region. 

 The term pseudopodium is applied to such a projection, and 

 the motion itself is usually known as amoeboid motion, from 

 its being specially developed in an extremely simple animal 

 organism known as Amoeba, the discussion of whose cha- 

 racters will occupy us later on. The mass of the protoplasm 

 may follow the pseudopodium, and locomotion of the cell 

 be effected. Definite motion, or motion always in one and 

 the same direction, occurs only in cells which are more 

 differentiated than those we are at present discussing (e.g. 

 muscle). Reference to definite motion is, therefore, post- 

 poned. 



Again, protoplasm is irritable that is to say, if a stimulus 

 be applied to the mass, such as a shock of electricity, an 

 application of gentle heat, or certain chemical substances 

 the protoplasm responds, shows irritability or excitement, 

 local or general. For example, a shock of electricity 



