20 INTRODUCTION. 



will, therefore, contain substances representing various stages of becoming 

 living, and of ceasing to be living, as well as the living substance itself. 

 And we may safely make this statement though we are quite unable to 

 draw the line where the dead food on its way up becomes living, or the 

 living substance on its way down becomes dead. 



5. Nor is it necessary for our present purpose to be able to point out 

 under the microscope, or to describe from a histological point of view, the 

 parts which are living and the parts which are dead food or dead waste. 

 The body of the amoeba is frequently spoken of as consisting of " proto- 

 plasm." The name was originally given to the matter forming the primor- 

 dial utricle of the vegetable cell as distinguished from the cell wall on the 

 one hand, and from the fluid contents of the cell or cell sap on the other, 

 and also we may add from the nucleus. It has since been applied very 

 generally to such parts of animal bodies as resemble, in their general fea- 

 tures, the primordial utricle. Thus the body of a white blood-corpuscle, or 

 of a gland cell, or of a nerve cell, is said to consist of protoplasm. Such 

 parts of animal bodies as do not in their general features resemble the 

 matter of the primordial utricle are not called protoplasm, or, if they at 

 some earlier stage did bear such resemblance, but no longer do so, are some- 

 times, as in the case of the substance of a muscular fibre, called " differen- 

 tiated protoplasm." Protoplasm in this sense sometimes appears, as in the 

 outer part of most amoebae, as a mass of glassy-looking material, either con- 

 tinuous or interrupted by more or less spherical spaces or vacuoles filled 

 with fluid, sometimes as in a gland cell as a more refractive, cloudy-looking, 

 or finely granular material arranged in a more or less irregular network, or 

 spongework, the interstices of which are occupied either by fluid or by some 

 material different from itself. We shall return, however, to the features of 

 this " protoplasm " when we come to treat of white blood-corpuscles and 

 other " protoplasmic" structures. Meanwhile it is sufficient for our present 

 purpose to note that lodged in the protoplasm, discontinuous with it, and 

 forming no part of it, are in the first place collections of fluid, of watery 

 solutions of various substances, occupying the more regular vacuoles or 

 the more irregular spaces of the network, and in the second place discrete 

 granules of one kind or another, also forming no part of the protoplasm 

 itself, but lodged either in the bars or substance of the protoplasm or in the 

 vacuoles or meshes. 



Now, there can be little doubt that the fluids and the discrete granules are 

 dead food or dead waste, but the present state of our knowledge will not 

 permit us to make any very definite statement about the protoplasm itself. 

 We may probably conclude, indeed we may be almost sure, that protoplasm 

 in the above sense is not all living substance, that it is made up partly of 

 the real living substance, and partly of material which is becoming living or 

 has ceased to be living; and in the case where protoplasm is described as 

 forming a network, it is possible that some of the material occupying the 

 meshes of the network may be, like part of the network itself, really alive. 

 " Protoplasm " in fact, as in the sense in which we are now using it, and shall 

 continue to use it, is a morphological term ; but it must be borne in mind that 

 the same word " protoplasm " is also frequently used to denote what we have 

 just now called " the real living substance." The word then embodies & phy- 

 siological idea ; so used it may be applied to the living substance of all living 

 structures, whatever the microscopical features of those structures ; in this 

 sense it cannot at present, and possibly never will be recognized by the 

 microscope, and our knowledge of its nature must be based on inferences. 



Keeping then to the phrase " living substance " we may say that each 

 piece of the body of the amoeba consists of living subs-tan ce, in which are 



