HISTOLOGY. 



be grouped the response of cells to stimuli of various sorts such as heat, 

 light, electricity, chemical reagents, the nervous impulse, or mechanical 

 interference. Metabolism, in a wide sense, includes the ingestion and as- 

 similation of food, the elaboration and secretion of desirable products, 

 together with the elimination of waste products. Contractility may 

 be manifest in the locomotion of the entire cell, in the vibratile action of 

 slender, hair-like processes, the cilia, or in contraction of the cell body. 

 Conductivity is the power of conveying impulses from one part of the cell 

 to another. Reproduction is seen in the process of cell division. Many 

 phases of these activities are observed in microscopic sections and as such 

 they will be referred to in later chapters. A few which are of general 

 occurrence will be described presently. 



AMOEBOID MOTION. 



The unicellular animal, Amoeba, exhibits a type of motility known 

 as amoeboid, which has been observed in many sorts of cells in the verte- 

 brate body. In marked cases, as 

 o % i 2 2' 2 in certain white blood corpuscles 



!i /? *a ^3 ( tne leucocytes), the cell protoplasm 



' & ^^^^ ^^^^ 



** sends out fine or coarse processes 



which divide or fuse with one an- 

 other, causing the cell to assume a 

 < great variety of forms. The proc- 



10 Minutes. & J 



esses may be retracted, or they may 



FIG. 8. LEUCOCYTES OF A FROG. X 560. 

 Changes in form observed during ten minutes- become attached Somewhere and 



I'. aUai?mi n K. etc. * draw the remainder of the cell body 



after them, the result of which is 



locomotion or the so-called wandering of the cell. Such wandering cells 

 play an important part in the economy of the animal body. Their proc- 

 esses can flow around granules or cells and thus enclose them in proto.- 

 plasm. 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 preparations from warm- 

 blooded animals they may be accelerated by gently heating the object. 



Another form of motion, which, however, does not occur in living 

 cells, consists in an oscillation of minute granules within the cell. This 

 may be due to diffusion currents or to the Brownian phenomena. It 

 may often be seen in salivary corpuscles. 



