STRUCTURAL CHARACTERS OF ANIMAL ORGANISMS. 55 



or united by long protoplasmic processes. This is the mucous tis- 

 sue common in the lower animals, in textures of the embryo in the 

 adult and in pathological growths of the connective-tissue type. 

 The processes uniting the cells may not be present, and the cells 

 may be reduced to a minimum, as occurs in the vitreous humor 

 of the eye. But more commonly the soft, gelatinous substance is 

 reduced in amount, and the processes connecting the cells are con- 

 verted into a dense network of delicate threads to form the reti- 

 form tissue of lymphoid structures. 



Fibrous Tissue. The cells may become further differen- 

 tiated and fibrillated. When the thickness of the cell processes is 

 great, and their fibrillation well marked, the cells appear to devote 



F.o. 20. 



Portion of tendon from the tail of a young rat, stained with gold chloride, showing 

 arrangement of flattened cells on bundles of fibrils. (After Klein.) 



all their reproductive energy to the formation of this fibrillated 

 substance which ultimately forms the great bulk of the tissue, 

 while the cells become gradually and proportionately fewer in 

 number. In this case only sufficient of the mucous substances 

 generally remains to cement the fibrils together into bundles. A 

 few of the cells, however, remain between the bundles of fibrils 

 to preside over the nutrition of the tissue. Thus is formed the 

 non-elastic or white fibrous tissue of tendon. 



As a general rule, these fibrils are easily affected by chemical 

 reagents. Weak acids cause them to swell up and become in- 

 distinct. Baryta water affects the cement and renders them 



