60 HISTOLOGY 



work of branching cells, in the meshes of which there is a homogeneous, 

 fluid, intercellular substance. The intercellular portion of the tissue 

 becomes highly developed and variously modified. 



Although typical epithelium and mesenchyma are radically different, 

 as shown in Fig. 31, p. 42, there are conditions in which they are com- 

 parable. Thus dense mesenchyma, in which the cells are closely packed 

 and have very little intercellular substance, resembles epithelium, and it 

 may give rise to groups or cords of epithelioid cells. Moreover epithe- 

 lium may resemble mesenchyma by forming a vacuolated syncytium, or 

 as seen in Fig. 49, a branching protoplasmic network. In epithelium the 

 intercellular spaces arise as vacuoles in the exoplasm, and the inter- 

 cellular substance of mesenchyma may also be considered as occupying 

 coalescent vacuoles. 



Intercellular spaces. 



Nuclei 



Intercellular bridges. 

 PIG. 49. FLAT EPITHELIAL CELLS FROM THE BRANCHIAL PLATE OF A LARVAL SALAMANDER. X 300 



The tissue of the adult which most closely resembles mesenchyma is 

 known as reticular tissue. It cannot, however, be regarded as an imma- 

 ture connective tissue, or a persistence of the primitive mesenchyma, 

 since it arises rather late in embryonic development (e.g., in the lymph 

 glands which first appear hi human embryos measuring about 45 mm., 

 and in the oesophagus of embryos of 30 mm.). It is therefore considered 

 to be a special form of connective tissue. 



RETICULAR TISSUE. 



Reticular tissue forms the framework of lymph glands, red bone 

 marrow and the spleen; it occurs as a layer immediately beneath the 

 epithelium of the digestive tract, and has been reported in many other 

 organs. It consists of a network of cells in relation with an abundant 



