8 



ANIMAL BIOLOGY 



final analysis, the product of the life of the component cells: the 

 expression of their harmonious cooperation. Accordingly the cells 

 are the basic units of the actual living matter, termed protoplasm. 



\b 



Fig. 3. — Vertical section (highly magnified) of a leaf to show its cellular 

 structure, a, guard cells, at opening (stoma) through epidermis; 6, cells con- 

 taining chlorophyll; c, upper and lower epidermal cells. (From Bailey.) 



Such being the case, we reach the first great biological general- 

 ization: all animals and plants have the same elementary cellular 

 structure. Indeed, the specific local differentiations in the living 



Fig. 4. — Transverse section (highly magnified) of a simple animal (Hydra) 

 to show the cellular structure. Outer layer, ectoderm; inner layer, endoderm; 

 central cavity, enteric cavity. 



materials of the various parts of animals and plants are made 

 possible largely because the protoplasm is disposed in microscopic 

 unit masses, or cells. 



