ITS CHEMICO-PHYSICAL AND MORPHOLOGICAL PROPERTIES 27 



or, according to their significance, as reserve material and secretions, 

 or indeed to specify them, as yolk granules, fat globules,, starch 

 granules, pigment granules, etc. 



The difference between the protoplasm and these substances, 

 which may be classed together as cell contents, is the same as that 

 between the materials of which the organs of our body are com- 

 posed and those substances which in the first place are taken up 

 as food by our bodies, and which later on are circulated in a 

 liquid form as a nutrient fluid through all the organs ; the for- 

 mer, which are less dependent upon the condition of nourishment 

 of the body for the time being, and hence are less subject to 

 variations, are called in physiological language tissue substances, 

 the latter circulating substances. The same distinction may be 

 applied to the substances which compose the cell. Protoplasm 

 is the tissue material, whilst the adventitious bodies are circulating 

 substa?ices. 



f. Various examples of the structure of the cell body. 

 In connection with the chemico-physical and morphological pro- 

 perties of the cell, a few especially pertinent examples may be 

 of use in order to explain the general statements. For this pur- 

 pose we will compare various lower unicellular organisms, both 

 plant and animal, choosing first, cases in which the body consists 

 almost entirely of protoplasm, and secondly, those in which the 

 cells also contain considerable quantities of various adventitious 

 substances, and hence are very much altered in appearance. 



Unicellular organisms, which live in water or on damp earth, 

 such as Amceba3, Mycetozoa, and Reticularia, form very useful 

 subjects for examination in studying the cell ; in addition, lymph 

 corpuscles, the white blood corpuscles of vertebrates, and young- 

 plant cells are most suitable objects for investigation. 



1. Cells consisting almost entirely of Protoplasm. An Amoeba 

 (Fig. 7) is a small mass of protoplasm, from the surface of 

 which, as a rule, a few short irregular processes (pseudopodia) 

 or foot-like organs are extended. The body is quite naked, that 

 is to say, it is not separated from the surrounding medium by 

 any special thin coating or membrane ; the only differentiation 

 being that the superficial layer of the protoplasm (ectoplasm), ek, 

 is free from granules, and hence is transparent, like glass ; this 

 ectoplasm is most marked in the pseudopodia; below the ectoplasm 

 lies the darker and more liquid endoplasm (en), in which the 

 vesicular nucleus (n) is embedded. 



