34 



CELL DIFFERENTIATION AND THE ELEMENTARY TISSUES 



mals, such as the frog, they are found widely distributed not only in the skin, 

 but also in internal parts, the mesentery, sheaths of blood vessels, etc. Under 

 the action of light, electricity, and other stimuli, the pigment granules become 

 massed in the body of the cell, leaving the processes quite hyaline; if the 

 stimulus be removed, they will gradually be distributed again throughout 

 the processes. Thus the skin in the frog is sometimes uniformly dusky and 

 sometimes quite light colored, with isolated dark spots. 



Intercellular Substance. This isjibrillar, as in the fibrous tissues and in 

 certain varieties of cartilage; or homogeneous, as in typical mucoid tissue. 



The fibers composing the former are of two kinds, white fibrous and 

 yellow elastic tissue. 



FIG. 38. 



FIG. 38. Mature White Fibrous Tissue of Tendon, Consisting Mainly of Fibers with a 

 Few Scattered Fusiform Cells. (Strieker.) 



FIG. 39. Caudal Tendon of Young Rat, Showing the Arrangement, Form, and 

 Structure of the Tendon Cells. X 300. (Klein.) 



The chief varieties of connective tissues may be thus classified: 



White fibrous. 



Elastic. 



Areolar. 



Gelatinous. 



Adenoid or retiform. 



Adipose. 



Neuroglia. 



Cartilage. 



1. Hyaline. 



2. White fibrous. 



3. Elastic. 

 Bone and dentine. 



The White Fibrous Tissue. It is found typically in tendon; also in 

 ligaments, in the periosteum and perichondrium, the dura mater, the peri- 



