PLATE 3 



EXPLANATION OF FIGURES 



4 Portion of transverse section of 11 mm. embryo of R. sylvatica. X 760. 

 Mallory connective tissue stain. The region shown lies just below the notochord 

 as in figure 3. The connective tissue ground substance (G.S.) contains a num- 

 ber of wandering mesenchyme cells which take the same stain as do the sur- 

 rounding muscle fibers (M.F.) — both types of cells being clearly differentiated 

 by the stain from the secreted connective tissue ground substance which is 

 formed before any cells are present in it. 



472 



