VARIETIES OF CONNECTIVE TISSUE 39 



with one another into a network or sponge-work by their processes ; 

 in some cases, the cell-nuclei have disappeared (as at b, fig. 45). 



FIG. 45. THIN SECTION FROM THE CORTICAL PART OF A LYMPHATIC GLAND, 



MAGNIFIED. 



A network of fine trabeculae formed by retiform tissue, from the meshes of which the 

 lymph-corpuscles hare been washed out, except at c, where they are left. 



Lymphoid or adenoid tissue is retiform tissue in which the meshes 

 of the network are largely occupied by lymph -corpuscles. This is by 

 far the most common condition of a retiform tissue, and is met with in 

 the lymphatic glands and allied structures (see Lesson XXII.), and also 

 in the tissue of the alimentary mucous membrane, and in some other 

 situations. 



Basement membranes (membranae propriae) are homogeneous-look- 

 ing membranes, which are found forming the surf ace -layers of con- 

 nective-tissue expansions in many parts, especially where there is a 

 covering of epithelium, as on mucous membranes, in secreting glands, 

 and elsewhere. They are generally formed of flattened connective- 

 tissue cells joined together to form a membrane ; but, in some cases, 

 they are evidently formed not of cells, but of condensed ground- 

 substance, and in others they are of an elastic nature. 



Jelly-like connective tissue, although occurring largely in the em- 

 bryo, is found only in one situation in the adult viz. forming the 

 vitreous humour of the eye. It seems to be composed entirely of soft 

 ground-substance, with cells scattered here and there through it, and 

 with very few fibres, or none at all. These several varieties of con- 

 nective tissue will be more fully described in connection with the 

 organs where they occur. 



Development of connective tissue. Connective tissue is always 

 developed in the mesoblast or mesoderm of the embryo. In those 

 parts of this layer which are to form connective tissue, the embryonic 

 cells become separated from one another by a muco-albuminous semi- 

 fluid intercellular substance (ground- substance), but the cells generally 

 remain connected by their processes. The connective-tissue fibres, 



