32 



MANUAL OF PHYSIOLOGY. 



FIG. 6. 



Modifications of Original Cell Tissues. In the pre- 

 ceding pages the special characters of a single cell have been dwelt 

 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 va- 

 rious 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. 



a FIG. 7. 



Unicellular organism. Small 

 amoeba. (Cadi at.) 



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

 a multiple mass by division. (Gegenbauer.) 



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. This group, called 

 Protitta, though insignificant in point of its size, may vie with the 

 higher plants and animals in number, species, and variety of form, 



