THE PRINCIPLES OF BIOLOGY 



and a cell may be defined as a bit of protoplasm containing 

 a nucleus. This definition includes naked cells as well as 

 cells with walls. 



One-celled Animals. There are countless millions of 

 animals and plants the existence of which was not sus- 

 pected until the invention of the micro- 

 scope several centuries ago. They are 

 one-celled, and hence microscopic in size.' 

 It is believed that the large animals and 

 plants are descended from one-celled ani- 

 mals and plants. In fact, each individual 

 plant or animal begins life as a single 

 cell, called an egg cell, and forms its 

 organs by the subdivision of the egg cell into many cells. 

 An egg cell is shown in Fig. 6, and the first stages in the 

 development of an egg cell are shown in Fig. 7. 



The animals to be studied in the first chapter are one- 

 celled animals. To understand them we must learn how 



FIG. 6. Egg cell of 

 mammal with yolk. 



FIG. 7. Egg cell subdivides into many cells forming a sphere (morula) containing 

 a liquid. A dimple forms and deepens to form the next stage (gastrula). 



they eat, breathe, feel, and move. They are called Pro- 

 tozoans (Greek protos, first; zoon, life). All other animals 

 are composed of many cells and are called Metazoans 

 (Greek meta t beyond or after). The cells composing the 

 mucous membrane in man are shown in Fig. 8. The cellu- 

 lar structure of the leaf of a many-celled plant is illustrated 

 in Fig. i. (See also Chap. I, Human Biology.) 



