166 



TEXT-BOOK OF EMBRYOLOGY. 



Fibrils and Fibers. The least differentiated and perhaps the least 

 specialized tissue derived from mesenchyme is reticular tissue, such as that 

 found in the lymph nodes and spleen. In the peripheral part of the cyto- 

 plasm, or exoplasm, of the mesenchymal cells and their processes there arise 

 delicate fibrils, often extending from one cell to another, which probably 



FIG. 145. Fibril forming cells from fresh subcutaneous tissue of head of chick embryo. Boll. 



represent specialized parts of the spongioplasm. These fibrils maintain 

 their intracellular position instead of becoming separated from the parent 

 cytoplasm, so that the reticular tissue retains a marked resemblance in 

 form to the original mesenchyme. 



FIG. 146. Connective tissue (mesenchymal) cells from larval salamander. Flemming. 



The first step in the development of the true fibrillar forms of connective 

 tissue from mesenchyme is the formation of fibrils and fibers. While it has 

 been held by some investigators that the fibrils arise in and from the homo- 

 geneous intercellular substance, the best substantiated view is that they arise 

 within and from the cytoplasm of the mesenchymal cells (Figs. 145 and 146). 

 They then become separated from the cytoplasm and lie free in the " ground" 



