PLATE 4 



EXPLANATION OF FIGURES 



23 Low power sketch of operculimi and filament froin a chick embryo of 4 

 days and 23 hours (H. E. C, Ser. 2059; 14.0 mm.; section 1004) X 50 diam. 



24 High power sketch from same section as figure 23 sliowing longitudinal 

 section of an opercular filament. Camera lucida drawing X 750 diam. ect., ecto- 

 derm covering the under surface of the operculum; mes., underlying mesenchyma. 



25 Epithelial cyst in process of formation. Camera lucida drawing of sec- 

 tion 207. X 900 diam. (H. E. C, Ser. 1954; 4 days, 3 hours; 9.0 mm.), ect., ecto- 

 derm covering the fourth branchial arch; mes., underlying mesenchyma. The 

 vesicle figured measures 19/1 in diameter. 



26 After Ekman's figure 29: "Horizontalschnitt durch die Kiemengegend 

 einer Bombinator — Larve 3 Tage nach der Entfernung der entodermalen Mund- 

 hohlemvand im I. Stadium. Bg. Blutgefjisse; KI-III 1-3. Kiemenrcihe; Op., 

 Operculum." X 200 diam. 



27 Section through the hyoid region of a 12.0 mm. chick (5 days, 1 hour, 

 X 100 diam.), for comparison between the normally occurring vestigial filaments 

 and opercular fold of the chick and the abortive filaments and peribranchial 

 chamber experimentally produced in toad embryos by Ekman (cf. with fig. 26). 



234 



