12 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



i. Size. — Cells are nearly always microscopic in 

 size, but are far more minute in animals than in plants. 



2. Softness. — In plants each cell is incased in a 

 firm shell of cellulose, so that in thin sections, such as 

 are used in microscopic examination, they retain their 

 form perfectly ; while in animals, unless specially pre- 

 pared, they collapse and lose their cellular appearance. 



3. Transparency. — The cells of animals, unless 

 specially prepared by staining, are so transparent that 

 their outlines are often detectable with difficulty. This 

 is far less true in plants. 



4. More highly Differentiated. — But with proper 

 care all these difficulties may be overcome. The real 

 greatest difficulty is the differentiation of cell form, 

 which is much greater in animals than in plants. Cells 

 take on different forms in order to perform different 

 functions. Therefore as functions increase in number 

 and become more perfect the cells take on more numer- 

 ous forms, and the forms differ more and more from one 

 another, and all from simple undifferentiated cells. Now 

 functions are more numerous and higher in animals than 

 in plants, and therefore the structure of animals, espe- 

 cially the higher animals, differs greatly from simple cellu- 

 lar structure ; so that in the mature condition of the high- 

 er animals the simple cellular structure may be entirely 

 lost. The universal cellular structure of animals there- 

 fore is best seen in the lowest animals and in the embry- 

 onic condition of the higher. 



TISSUES. 



A tissue may be defined as an aggregate of cells of 

 the same form and having the same function, but differing 

 in form and function from the cells of other aggrega- 

 tions; different tissues therefore are different stv/es of cell 

 structure, each adapted to a peculiar function. The ani- 



