PLATE 4 



EXPLANATION OF FIGURES 



21 Portion of a transverse section through the pharyngeal region of mero- 

 plast Type I, no. 31, near the plane of figure 14. 



22 Portion of a transverse section through the pharyngeal region of mero- 

 plast Type I, no. 122, showing conditions very similar to those in figure 21. The 

 endothelial tubes of the two sides are in communication ventral to the pharynx. 

 The endothelium "Stains more darkly than does the mesenchyme. 



ABBREVIATIONS 



Coel., coelom Par. mes., parietal mesoderm 



Endth., endothelium Vis. mes., visceral mesoderm 



Ent., entoderm 



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