In certain parts of the embryo and in some of the lower animals a kind of connective tissue is found in which there is but little intercellular substance, the mass of the tissue being thus made up of cells. The cellular connective tissue never forms an important texture in the adult, but is interesting as the probable tissue from which the connective tissues are formed in the embryo, and as occurring in abnormal growths or tumors.

The first step in its differentiation is the secretion of a large quantity of soft, homogeneous, semi-gelatinous or fluid material like the mucus secreted by epithelium. In this the cells lie, either free or united by long protoplasmic processes. 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 converted into a dense network of delicate threads to form the retiform tissue of lymphoid structures.

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

Fig. 26. 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).