PLATE 1 



EXPLANATION OF FIGURES 



Development of gill filaments in chick embryos of the fourth to the eighth day 



of incubation 



4 Chick; 3 days, 4 hours; 6.8 mm. X 9 diam. (H. E. C, Ser. 2057) I-IV, 

 first four ectodermal grooves (cf. with fig. 10). 



5 Chick; 3 days, 22 hours; 8.1 mm. X 9 diata. (cf. with fig. 11). Note hillock 

 on third arch. 



6 Chick; 4 days, 23 hours; 13.4 mm. X 9 diam. I, auditory groove; //, 

 III, second and third ectodermal grooves limiting the wedge out of which the 

 filaments are differentiating (cf. with figs. 18 and 19). 



7 Chick; 6 days, 3 hours; 16.9 mm. X 6 diam. II, site of the second ecto- 

 dermal groove, occupied by a ridge bearing filaments; S.P., pectoral groove. 



8 Chick; 6 days, 5 hours; X 6 diam. (cf. with figs. 1 and 20). 



9 Chick; 7 days, 1 hour; 19.7 mm. X 6 diam. 



228 



