32 CELL DIFFERENTIATION AND THE CONNECTIVE TISSUES 



observed in the cornea, in which they are arranged, layer above layer, parallel 

 to the free surface. They lie in spaces in the intercellular or ground substance, 

 which form by anastomosis a system of branching canals freely communicating, 

 figure 37. 



The flattened tendon corpuscles which are arranged in long lines or rows 

 parallel to the fibers belong to this class of cells, figure 39. 



These branched cells often contain pigment granules, giving them a dark 

 appearance; they form one variety of pigment cell. Pigment cells of this 



FIG. 37. Horizontal Preparation of the Cornea of Frog, Stained in Gold Chloride; showing 

 the network of branched corneal corpuscles. The ground substance is completely colorless. 

 X 400. (Klein.) 



kind are found in the outer layers of the choroid. In many of the lower ani- 

 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 some- 

 times quite light-colored, with isolated dark spots. 



Intercellular Substance. This is fibrillar, 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. 



The chief varieties of connective tissues may be thus classified: 



White fibrous. 



Elastic.' 



Areolar. 



Gelatinous. 



