124 THE SMALLEST LIVING THINGS 



Toxin : any of a class of poisonous substances formed as secretion products of 

 vegetable and animal organisms. They are distinguished from inorganic poisons 

 by their property of inducing the formation of an anti-toxin when introduced 

 into a suitable animal and, in most cases, by an incubation period before 

 symptoms of poisoning appear. 



Typical : combining or exhibiting the essential characteristics of a group. 



Vaccination : the inoculation with a weakened virus as a preventive measure 

 against disease. 



Vacuole : a small cavity containing a watery fluid ; it is characteristic of plant 

 cells and protozoa, but occurs also in cells of higher animals. In plants 

 vacuoles contain the cell sap ; in protozoa they may contain secretions of the 

 protoplasm, or substances about to be excreted, or food in various stages of 

 digestion and assimilation. 



Vacuole, Contractile: a vacuole found in the protoplasm of many unicellular 

 organisms, which gradually increases in size, and then suddenly collapses, 

 often making regular pulsations. Its function is believed to be respiratory 

 or excretory. 



Vacuolization : the state of containing one or more vacuoles. 



Ventral: designating, pertaining to, or situated on or toward that surface of 

 the body which in man is anterior, but which in most vertebrates and inverte- 

 brates is the lower surface — opposed to dorsal. 



Virus: the poison (contagium) of an infectious disease. 



Zoospores : reproductive bodies with the ability to move, usually by means of 

 flagella ; used by botanists for filamentous algae. 



Zygospore: a spore formed by the conjugation of two similar gametes; a term 

 used by botanists in the same sense as the term zygote (q.v.) is used by 

 zoologists. 



Zygot, or Zygote : the product of the union of two sexual cells ; a fertilized cell. 



