THE MULTICELLULAR ORGANISM 59 



endoderm forms the layer of cells which lines the alimentary 

 canal of the adult organism. 



This grouping of more or less similar cells into functional 

 systems, or tissues, is at the basis of the architecture of 

 multicellular organisms, and thus we have now reached 

 another level in the analysis of their structure. Although 

 the unit of organization is the cell, these are associated in 

 groups, or tissues, which represent a morphological unit of a 



FIG. 20. Portion of a cross section of the small intestine of the Frog, 

 highly magnified to show cellular differentiation and tissues, bl, blood 

 vessels; cq, unicellular glands; ep, ordinary absorptive epithelial cells 

 lining the intestine; es, connective tissue; m.c., circular muscle cells; 

 m. I., longitudinal muscle cells; pe, peritoneum. (From Holmes, after 

 Howes.) 



higher order. A TISSUE may be defined as a group of 

 essentially similar cells specialized to perform a certain func- 

 tion. Examples are bone, muscle, and nerve in animals; 

 and wood and bark in plants. (Figs. 20, 21.) 



Since the similar cell components of multicellular organisms 

 are grouped to form tissues, it follows that the major working 

 units, or ORGANS, of the animal or plant body as a whole are 

 formed of tissues. In other words an organ is a complex of 



