OF PROTOPLASM. 



1. It is contractile. There can be little doubt that the 

 changes in the protoplasm of an amoeba which bring about its 

 peculiar ' amoeboid ' movements, are identical in their fundamental 

 nature with those which occurring in a muscle cause a contraction : 

 a muscular contraction is essentially a regular, an amoeboid move- 

 ment an irregular flow of protoplasm. The substance of the 

 amoeba may therefore be said to be contractile. 



2. It is irritable and automatic. When any disturbance, 

 such as contact with a foreign body, is brought to bear on the 

 amoeba at rest, movements result. These are not passive move- 

 ments, the effects of the push or pull of the disturbing body, 

 proportionate to the force employed to cause them, but active 

 manifestations of the contractility of the protoplasm ; that is to 

 say, the disturbing cause, or ' stimulus/ sets free a certain amount 

 of energy previously latent in the protoplasm, and the energy set 

 free takes on the form of movement. Any living matter which, 

 when acted on by a stimulus, thus suffers an explosion of energy, 

 is said to be ' irritable.' The irritability may, as in the amoeba, 

 lead to movement ; but in some cases no movement follows the 

 application of the stimulus to irritable matter, the energy set free 

 by the explosion taking on some other form than movement, 

 ex. gr. heat. Thus a substance may be irritable and yet not 

 contractile, though contractility is a very common manifestation 

 of irritability. 



The amoeba (except in its prolonged quiescent stage) is rarely 

 at rest. It is almost continually in motion. The movements 

 cannot always be referred to changes in surrounding circumstances 

 acting as stimuli ; in many cases the energy is set free in conse- 

 quence of internal changes, and the movements which result are 

 called spontaneous or automatic movements. We may therefore 

 speak of the protoplasm of the amoeba as being irritable and 

 automatic. 



3. It is receptive and assimilative. Certain substances 

 serving as food are received into the body of the amoeba, and there 

 in large measure dissolved. The dissolved portions are subse- 

 quently converted from dead food into new living protoplasm, and 

 become part and parcel of the substance of the amoeba. 



4. It is metabolic and secretory. Pari passu with the re- 

 ception of new material, there is going on an ejection of old 

 material, for the increase of the amoeba by the addition of food is 

 not indefinite. In other words, the protoplasm is continually 

 undergoing chemical change (metabolism), room being made for 

 the new protoplasm by the breaking up of the old protoplasm into 

 products which are cast out of the body and got rid of. These 

 products of metabolic action have, in many cases at all events, 

 subsidiary uses. Some of them, for instance, we have reason to 

 think, are of value for the purpose of dissolving and effecting other 



