300 MICROBES AND TOXINS 



Carbohydrates — compounds of the three elements, carbon, oxygen, 

 and hydrogen . the chief representatives are the sugars and 

 starches. 



Catalysts— substances which modify the rapidity of a reaction 

 without forming part of its final products : such a reaction is 

 termed "catalysis'" (Berzelius, 1835): for example peroxide of 

 hydrogen, H2O2, is decomposed into water, and oxygen on 

 contact with spongy platinum, or platinum in a state of fine 

 division. The platinum remains unaltered : it has acted 

 merely by its presence. 



Chemiotaxis — a sort of " chemical sense " by means of which living 

 cells seem to seek the substances favourable to them, and 

 avoid those which are injurious. Positive and negative 

 'chemiotaxis are indicated by movements of attraction and 

 repulsion respectively, i.e., by approach or flight. 



Chromatin — the substance composing the greater part of the 

 nucleus of cells, so-called because it stains very deeply with 

 certain dyes. 



Chromidia — masses or networks of chromatin distinct from the 

 nucleus itself. 



Colloids — Graham gave the name of " colloids " to those substances 

 which in watery solution do not dialyse (i.e., do not pass 

 through a parchment membrane dipping in pure water), or 

 dialyse extremely slowly in contrast to the " crystalloid " 

 substances which rapidly dialyse. Types of these two classes 

 are "gum and salt. Nowadays it is more usual to speak of 

 "substances in the colloidal state" than of "colloidal sub- 

 stances." The " colloidal state " is a state of suspension of one 

 substance in another as contrasted with true solution. For 

 example gutta-percha forms a true solution in alcohol, a 

 colloidal solution in water. In the animal body the cells, the 

 cell-membranes, and the body-fluids all consist of colloidal 

 solutions, the condition and activities of which are regulated 

 by physico-chemical laws : the colloids are thus of extreme 

 interest to biologists. 



Conidium — a spore of a fungus produced asexually and borne on a 

 special branch. 



Cytoplasm — the protoplasm of the cell-body as distinguished from 

 the nucleus, the protoplasm of which is known as nucleo- 

 plasm. 



Daphnia — the common water-flea. 



Dialysis — vide Colloids. 



Diapedesis — a phenomenon discovered by Cohnheim : the white 

 corpuscles of the blood (leucocytes) emigrate from the interior 

 of the blood-vessels by a process of active movement not 

 passive expulsion. They insinuate themselves between the 

 cells forming the walls of the capillaries, and drag themselves 

 through as narrow threads to recover their rounded shape 

 outside. 



