INTRODUCTION. 5 



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 'proto- 

 plasm ' when we come to treat of white blood corpuscles and other 

 ' protoplasmic ' structures. Meanwhile it is sufficient for our pres- 

 ent 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 a physiological idea ; so used it may be applied to 

 the living substance of all living structures, whatever the micro- 

 scopical features of those structures ; in this sense it cannot at 

 present, and possibly never will be recognised 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 

 substance in which are lodged, or with which are built up in 

 some way or other, food and waste in various stages. 



Now, an amoeba may divide itself into two, each half exhibiting 

 all the phenomena of the whole ; and we can easily imagine the 

 process to be repeated until the amoeba was divided into a 

 multitude of exceedingly minute amoebae, each having all the 

 properties of the original. But it is obvious, as in the like 

 division of a mass of a chemical substance, that the division could 

 not be repeated indefinitely. Just as in division of the chemical 

 mass we come to the chemical molecule, further division of which 

 changes the properties of the substance, so in the continued 

 division of the amoeba we should come to a stage in which further 

 division interfered with the physiological actions ; we should come 



