PLATE 2 



EXPLANATION OF FIGURES 



3 Portion of 9 mm. embryo of R. sylvatica. X 322. The ground substance 

 (G.S.) is shown lying below the notochord (N.C.) and extending laterally on 

 either side between the muscle plates to the body wall. At this stage some of 

 the mesenchyme cells are beginning to move into the ground substance. 



6 Portion of a transverse section through the tail region of a 75 mm. embryo 

 of R. eatesbiana. X 75. The figure shows a portion of the notochord sur- 

 rounded by the connective tissue ground substance (G.S.). In this late stage 

 the ground substance, particularly around the notochord (N.C), is largely com- 

 posed of bundles of typical connective tissue fibers. 



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