PLATE 1 



EXPLANATION OF FIGURES 



1 Portion of sagittal section of 3.2 mm. embryo of R. palustris. X 285. 

 The notochord (N.C. ) is shown imbedded in the cell-free connective tissue ground 

 substance (G.S.). The latter extends laterally on either side into the muscle 

 plates. 



2 Portion of transverse section of 4 mm. embryo of R. palustris. X 322, 

 A section of the notochord (N.C.) is shown imbedded in the connective tissue 

 ground substance (G.S.). 



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