46 



MANUAL OF PHYSIOLOGY. 



there is but little intercellular substance, the mass of the tissue 

 being thus made up of cells. This cellular connective tissue never 

 forms an important texture in the adult man, but is interesting 

 as the probable tissue from which all the connective tissues are 

 formed in the embryo, and as occurring in abnormal growths or 

 tumors. 



The first step in differentiation is the secretion of a large quan- 

 tity of soft homogeneous, semi-gelatinous or fluid material like the 

 mucus secreted by epithelium. In this the cells lie, either free 



FIG. 24. 



Transverse section of the chorda dorsalis and neighboring substance, a, 

 cartilage cells; 6, cell of the middle layer of embryo; c, mucous tissue; d, 

 boundary of chorda. (Cadiat.) 



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 retiform 

 tissue of lymphoid structures. 



