THE VITAL PROPERTIES OF THE CELL 



145 



species of micro-organisms appears to vary 

 considerably in different animals, and to 

 depend largely upon the most varying 

 conditions ; for instance, chemical stimuli 

 play an especially important part, as has 

 been already mentioned on p. 121 (negative 

 and positive chemotropism ; Hertwig IV. 

 13). Apparently it is upon this that the 

 greater or less immunity of organisms 

 from many infectious diseases depends. 

 This discovery opens a wide vista in the 

 field of the comprehension and treatment 

 of infectious diseases. 



IT. The Assimilative and Forma- 

 tive Activity of the Cell. The gases, 

 the fluids, and the solid substances, which 

 are introduced into the protoplasm as food, 

 and through respiration, compose the very 

 varying raw materials which are elaborated 

 in the chemical workshop of the cell, and 

 which are converted into an exceedingly 

 large number of substances. Amongst 

 these the most important for both plants 

 and animals are : carbo-hydrates, fats, pro- 

 teids, and their numerous compounds. 



Similarly the ways in which they are utilised in the vital pro- 

 cesses of the cell vary very considerably. They serve partly to 

 replace the substances, which, during the vital process, become 

 decomposed in the cell, such as the substance which is oxidised 

 during respiration, and which thus furnishes the vital energy 

 necessary for the activity of the cell. They are also utilised for 

 that growth and increase of the protoplasm which is absolutely 

 indispensable for the function of reproduction. Further, some 

 of the substances formed in the chemical laboratory are stored up 

 for future use in the cell-body in some form or other, thus consti- 

 tuting reserve material. Finally they may be set aside to fulfil 

 some function inside or outside the cell. 



Thus arise the different materials which, especially in the 

 animal kingdom, are very numerous, and upon which the dif- 

 ferentiation of tissues depends : glandular secretions, which are 

 passed to the exterior, membranes, and intercellular substances of 



L 



FIG. 61. A leucocyte of 

 a Frog, enclosing a Bac- 

 terium, which is undergo- 

 ing digestion. The Bac- 

 terium is stained with 

 vesuvine. The two figures 

 represent two stages of one 

 and the same cell. (After 

 Metchnikoff, Fig. 54.) 



