THE CELL-BODY. 6 1 



Inside of the cell-body the protoplasm also shows phenomena of 

 motion, the streaming of the protoplasm. In plant cells there is 

 often a noticeable regularity in the direction of the current. Men- 

 tion should not be omitted of the so-called molecular or Brownian 

 movement in the cells, which consists in a rapid whirling motion 

 of particles or granules suspended in the protoplasm (Brown). 



Living protoplasm is irritable in the highest degree, and reacts 

 very strongly to chemic and physical agents. It is very sensitive to 

 changes in temperature. All the phenomena of life occur in greater 

 intensity and more rapidly in a warm than in a cold temperature, 

 this fact being very strikingly shown by the phenomena of motion 

 in the cell, as also in its propagation. By subjecting protoplasm to 

 different temperatures, its various movements can be slowed or 

 quickened. It dies in too high or too low a temperature. 



Certain substances coming in contact with the cell from a given direc- 

 tion have on it an attracting or repelling action. These phenomena are 

 known as positive and negative chemotropism (chemotaxis) . The action 

 of chemic agents on the different wandering cells of the body and on cer- 

 tain free-swimming unicellular organisms naturally varies to a great 

 degree. Among these phenomena must be included those produced by 

 water (hydrotropism) and light (heliotropism). It is very probable that 

 all these phenomena are of importance to the proper appreciation of some 

 of the processes going on in the vertebrate body (as, for instance, in the 

 origin of diseases caused by micro-organisms). 



Protoplasm may contain various structures. Of these, the 

 vacuoles deserve special mention. They are more or less sharply 

 defined cavities filled with fluid, and vary considerably in number 

 and size. The fluids that they contain differ somewhat, but are 

 always secreted by the protoplasm, and are, as a rule, finally emp- 

 tied out of the cell. As a consequence, vacuoles are best studied 

 where the function of the cell is a secretory one. Here they are 

 often large, and sometimes fill up the whole cell, the contents of 

 which are then emptied out (glandular cells). 



Contents of a solid nature, such as fat, pigment, glycogen, and 

 crystals, are peculiar to certain cells. By these deposits the 

 cell is more or less changed, the greatest variation in form taking 

 place in the production of fat. The latter, as a rule, takes the shape 

 of a globule, and greatly modifies the position of the normal con- 

 stituents of the cell. Deposits of pigment alter the cells to a less 

 degree. This substance occurs in the protoplasm either in solution 

 or in the form of fine crystalline bodies. Glycogen is more gener- 

 ally diffused, occurring very generally in embryonal cells and in the 

 liver- and cartilage-cells of the adult. Occasionally we find larger 

 crystals in animal cells, as, for instance, in the red blood-corpuscles 

 of the teleosts. So-called margarin crystals sometimes occur in 

 large numbers as stellate figures in dead fatty tissues kept at low 

 temperatures. 



