THE BUILDING-STONES OF THE ORGANIC WORLD 



71 



In the living state the cell represents a minute drop of light- 

 coloured, almost opaque liquid. Like every other drop of liquid, 

 this cell has in the free state the desire to contract itself into the 

 smallest possible volume, that is, to assume globular form. The 

 sphere is theoretically the ideal form of the cell, but we find it 

 only rarely realized in Nature. Even the simplest protozoa 

 assume the form of a sphere only temporarily when about to 

 rest. In the body of the higher 

 organisms there is hardly a shape 

 which the cell is not able to 

 assume. Here we find cubic and 

 flat, cylindrical and oval, spindle- 

 shaped and branched cells, and 

 even in the unicellular organisms 

 the variety of forms is astonishing, 

 But however different the various 

 species may appear, however 

 radical the difference may be in 

 the form of the protozoon and 

 the unicellular Alga or the cell of 

 the metazoon, there is one point 

 in which their structure always 

 agrees : their composition of protoplasm and nucleus. 



The most important and largest part of the cell, the substance 

 with which all functions of life appear to be connected, is the 

 protoplasm. But when we ask what this protoplasma really is 

 we are regretfully compelled to admit that at present we know 

 very little of its nature. 



Chemically, protoplasm is no homogeneous body, but a highly 

 complex compound in a continual state of change, consisting 

 mainly of albumen, which is formed chiefly of the five elements 

 hydrogen, carbon, oxygen, nitrogen, and sulphur. It is always 

 rich in water. When we withdraw the water, protoplasm 

 becomes hard and tough and the functions of life cease, without, 

 however, necessarily becoming extinct. We have already seen 

 that many cells are able in such a state of rest to remain alive 

 for a great number of years and, on a renewed supply of water, 

 to awaken to renewed activity. 



In slightly magnified protoplasm we can easily distinguish 



FIG. 20. STRUCTURE OF A CELL. 



