PLATE 4 



EXPLANATION OF FIGURES 



21 Lateral view of a model of the pharyngeal epithelium in a 14.5 mm. human 

 embryo (No. 1003, Harvard Embryological Collection). X 40. Other aspects 

 are shown in figures 18 and 22. 



22 Dorsal (posterior) view of the same model. X 40. 



23 Photograph of section No. 317 from a 12 mm. human embryo (No. 816, Har- 

 vard Embryological Collection), showing the Complex III cut longitudinally on 

 both sides. Upon the left side, parathyreoid III and the lumen of the complex, as 

 well as the ductus pharyngeo-branchialis III, are particularly well shown. Note 

 the relation of the ventral diverticulum (epithelial thymus) to the pericardium. 

 X 50; reduced 1/4. 



24 Photograph of section No. 417 from a 19 mm. human embryo (No. 828, 

 Harvard Embryological Collection), showing the Complex III cut nearly longi- 

 tudinally on both sides. Upon the left, parathyreoid III is particularly well 

 shown. X 30. 



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