PROTOPLASM. 363 



either physically mingled or perhaps weakly held together by the 

 feeble chemical affinities which belong to all massive molecules. This 

 constitutes a slime-like mass which, like all chemical compounds, ex- 

 hibits certain characteristic reactions by which it is clearly distinguish- 

 able from certain substances, and known to be closely allied to certain 

 others. 



Protoplasm has the power of absorbing water in varying quanti- 

 ties, so that it is sometimes soft and nearly fluid, and again hard and 

 leathery, though ordinarily of a medium consistence and density best 

 described by the term " slime-like " already employed. It is then a 

 glairy, tenacious, semi-fluid substance, transparent, and generally col- 

 orless ; and if not quite the homogeneous, structureless matter which 

 it was long supposed to be, there is at least an entire absence of differ- 

 entiation of structure quite comparable to the observed absence of lo- 

 calization of function. When it acts it acts en masse or indifferently, 

 sometimes in one portion, sometimes in another, of its substance, for 

 the production of its simple movements and for the bringing about of 

 its protean forms. Now all mouth, and anon all stomach, at times all 

 feet, and again all lungs, it fulfills Dryden's famous description, 

 " Everything by starts, and nothing long," save that it is ever and 

 always protoplasm. 



Like other albuminous substances, it is coagulable by heat, by al- 

 cohol, and by mineral acids, and is similarly stained by iodine and by 

 nitric acid. Living protoplasm possesses also certain fundamental 

 properties by which it may be distinguished from dead protoplasm. 

 Prominent among these properties, grouped under the single term 

 " vital," may be mentioned first, excitability, or, as it is more com- 

 monly called, " irritability" by which is meant the power of respond- 

 ing to a stimulus. An amoeba suddenly brought in contact with some 

 foreign body responds to the stimulus so received by certain charac- 

 teristic movements. 



The movements of protoplasm, however, can not always be thus 

 traced to some external exciting cause. Watching a specimen beneath 

 the microscope, portions of the mass may be seen to creep, or rather to 

 flow, slowly away in fine threads uniting with other threads from dif- 

 ferent parts of the same mass, thus forming an irregular net-work. Or 

 perhaps it thrusts out temporary feet indifferently from any part of its 

 surface by means of which it creeps slowly about, and it draws them 

 in again, returning to the somewhat globular shape which appears to 

 belong to its quiescent state. These movements are spontaneous ; that 

 is, they originate in the mass and result from the essential constitution 

 of this kind of matter ; therefore protoplasm is described as " auto- 

 matic " or self-acting, and even as having a will of its own. 



The internal changes which bring about these movements are be- 

 lieved to be identical with those which occur in muscle-tissue under 

 stimulation, producing the change of form in muscle-fiber known as 



