PRINCIPLES OF GENERAL PHYSIOLOGY 



amoeba or the leucocyte, it appears, even under the highest powers of the 

 ordinary microscope, as a clear, colourless, jelly-like stuff, not showing any 

 structure, but nevertheless keeping itself distinct from the fluid surrounding 



it, not mixing therewith, and also 

 capable of changing its form in 

 response to changes in its surround- 

 ings (see Fig. 1). fa 



The structureless nature of protoplasm 

 in its most elementary form is also, in 

 certain cases, to be seen after fixation, as 

 is shown in Fig. 2, in which it will be 

 noticed that the external layer and the 

 pseudopodia are completely clear. 



Even in some of the simplest 

 unicellular organisms special portions 

 are differentiated off for the purpose 

 of performing particular functions, 

 the contractile vacuole, for example. 



^^^^^^^^^^ ^^^^^^^^ In higher forms of life such parts 



are known as " organs," and exist as 

 permanent structures ; whereas the 



/ simpler creatures appear to possess 



the power of making organs as they 



FIG. 2. LEUCOCYTE OF NEWT. Fixed by a are required. The food vacuoles, 

 jet of steam directed on to the cover-glass. seen in Fig. 3, may be given as an 

 Stained with hiematoxylin. Untouched instance. The water, taken in 



photograph Note the apparently homo- with food par ticles, forms temporary 



geneous nature of the pseudopodial ., 



protoplasm. (Schafer, "Essentials of stomachs, as it were, into which 

 Histology," fig. 67, PI. 58.) digestive agents are secreted. 





** 



FIG. 3. DIXAMCEBA MIRABILIS. Interior filled with numerous 

 cells of an alga, Didymoprium, enclosed in drops of liquid. 

 The vacuoles are spherical, although the organisms included 

 have irregular shapes. Eachdivision of the scale corresix >nds 

 to 2-8 fM. (Leidy, 1879, PI. vii. fig. 3.) 



