THE STRUCTURE OF THE ELEMENTARY TISSUES. 41 



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

 skin, but also in internal parts, e.g., the mesentery and sheaths of blood- 

 vessels. In the web of the frog's foot such cells may be seen with pig- 

 ment granules evenly distributed throughout the bodyof the cell and 

 its processes; but under the action of light, electricity, and other stim- 

 uli, the pigment granules become massed in the body of the cell, leaving 

 the processes quite hyaline; if the stimulus be removed, they will grad- 

 ually 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. In the choroid and retina the pigment cells 

 absorb light. 



(b.) Amceboid Cells, of an approximately spherical shape; they have 

 a great general resemblance to colorless blood-corpuscles, with which 



Fig. 36. Horizontal preparation of the cornea of frog, stained in gold chloride; showing the 

 network of branched cornea corpuscles. The ground substance is completely colorless, x 400. 

 (Klein.) 



some of them are probably identical. They consist of finely granular 

 nucleated protoplasm, and have the property, not only of changing their 

 form but also of moving about, hence they are termed migratory. They 

 are readily distinguished from the branched connective-tissue corpuscles 

 by their free condition, and the absence of processes. Some are much 

 larger than others, and are found especially in the sublingual (gland of 

 the dog and guinea-pig, and in the mucous membrane of the intestine. 

 A second variety of these cells called plasma cells are larger than the 

 amoeboid cells, apparently granular, less active in their movements. They 

 are chiefly to be found in the inter-muscular septa, in the mucous and 

 sub-mucous coats of the intestine, in lymphatic glands, and in the omen- 

 turn. 



(B.) Intercellular Substance. This may be fibrillar, as in the 

 fibrous tissues, and in certain varieties of cartilage; or homogeneous, as 

 in hyaline cartilage. 



