PLASM 



in many when they are young, but grows subsequently. 

 We now distinguish between naked cells (gymnocytes) 

 and covered cells (thecocytes). As examples of naked 

 cells we have the amoebae, and many of the infusoria, 

 the spores of algae, the spermatozoa, and many animal 

 tissue-cells. 



The cell-covering (cytotheca) varies very much in size, 

 shape, composition, and chemical character, especially 

 in the rhizopods among the unicellular protists. The 

 tlint shells of the radiolaria and diatomes, the chalky 

 cells of the thalamophora and calcocytea, the cellulose 

 shells of the desmidiacea and syphonea, show the ex- 

 traordinary plasticity of the constructive cytoplasm {cf. 

 chapter viii.). Among the histona the tissue-plants are 

 remarkable for the infinite variety of shape and differ- 

 entiation of their cellulose capsules. The familiar prop- 

 erties of wood, cork, bast, the hard shells of fruit, etc., 

 are due to the manifold chemical modification and mor- 

 phological differentiation which the cellulose membrane 

 undergoes in the tissues of plants. This is less fre- 

 quently seen in the tissues of animals; but, on the other 

 hand, the intercellular and the cuticular matter play a 

 greater part in these. 



The intercellular matter, an important external 

 plasma-product, is formed by the social cells in the 

 tissues of the histona thrusting out in common firm 

 protective membranes. These protective structures are 

 very common among communities of protists, in the 

 form of masses of jelly, in which a number of cells of 

 the same kind are united ; such are the zoogloea of many 

 of the bacteria and chromacea, the common jelly-like 

 envelope of the volvocina and many diatomes, and the 

 globular cell-communities of the polycyttaria (or social 

 radiolaria). The chief part is played by intercellular 

 matter in the body of the higher animals, in the form of 

 mesenchyma-tissue ; the connecting tissue, cartilages, 



lo 145 



