40 



MANUAL OF PHYSIOLOGY. 



FIG. 6. 



on, and it has also been seen that a cell may become changed from 



its original form in order to fit it 

 more perfectly for some special pur- 

 pose. But it is necessary to consider 

 this latter fact more fully in order to 

 understand the relation of the vari- 

 ous tissues of the adult body to each 

 other. 



The first stage in the existence of 

 any organism, from the simplest 

 form of plant to man, is composed 



of a single cell (in animals called the ovum or egg), which 

 differs in no essential points of structure from an ordinary cell. 



Unicellular organism. Small amoeba 

 (Cadiat.) 



FIG. 7. 



Stages in the division of the egg cell (ovum), showing the production of a multiple mass 

 by division. (Gegenbaur.) 



There is, moreover, a class of organisms which never goes beyond 

 this one-celled stage, and the individuals pass their entire lifetime 

 in the state of a simple unicellular organism. The individuals 

 composing this group, called Protista, though insignificant in 

 point of size, may vie with the higher plants and animals in 

 number, species and variety of form, so that they might well 



