12 HISTOLOGY 



brate body. In marked cases, as in certain white blood corpuscles (the 

 leucocytes), the cell protoplasm sends out fine or coarse processes which 

 divide or fuse with one another, causing the cell to assume a great variety 

 of forms. The processes may be retracted, or they may become attached 

 somewhere and draw the remainder of the cell body after them, the result 

 of which is locomotion or the so-called wandering of the cell. Such wander- 

 ing cells play an important part in the economy of the animal body. 

 Their processes can flow around granules or cells and thus enclose them 

 in protoplasm. Some of these ingested bodies may be assimilated by 

 the cell as a result of complex chemical and osmotic reactions. Cells 

 which feed on foreign particles and can alter or digest them are known as 

 phagocytes. Amoeboid movements take place very slowly. In prepara- 

 tions from warm-blooded animals they may be accelerated by gently 

 heating the object. 



Another form of motion is that which occurs within the protoplasm 

 of fresh cells, whether living or dead, and consists in a rapid oscillation 

 of minute granules, due to diffusion currents. Although these movements 

 were first observed within protoplasm, it was soon shown that they oc- 

 curred when various inert particles were suspended in a liquid. Robert 

 Brown described the motion in 1828, in an essay entitled "On the General 

 Existence of Active Molecules in Organic and Inorganic Bodies," and 

 the phenomenon is called the molecular or Brownian movement. It 

 may often be seen in salivary corpuscles. 



FORMATION AND REPRODUCTION OF CELLS. 



In the past, two sorts of cell formation have been recognized, namely 

 the spontaneous generation of cells, and the origin of cells through the 

 division of pre-existing cells. According to the theory of spontaneous 

 generation it was once thought that animals as highly organized as intes- 

 tinal worms came into existence from the fermentation of the intestinal 

 contents. After this had been disproved, it was still thought that uni- 

 cellular animals arose spontaneously and that cells might be formed 

 directly from a suitable fluid, the cytoblastema. Something of the sort 

 may have occurred when life began, and it is the expectation of certain 

 investigators that conditions may yet be produced which shall lead to 

 the formation of organic bodies capable of growth and reproduction. At 

 present, however, only one source of cells is recognized the division of 

 existing cells. "Omnis cellula e cellula." A nucleus likewise can arise 

 only by the division of an existing nucleus; it cannot be formed from non- 

 nucleated protoplasm. 



