PLATE 2 



EXPLANATION OF FIGURES 



Sections illustrating early development of filaments 



10 Section through the branchial clefts of an embryo of 3 days and 4 hours 

 (H. E. C, Ser. 2057; 6.8 mm. ; section 266; X 27 diam.), illustrating the stage when 

 vesicles first appear in the branchial epithelium (cf. with fig. 4). I-IV ectoder- 

 mal grooves; op., operculum. 



11 Section of an embryo of 4 days and 4 hours (H. E. C, Ser. 2058; 11.0 mm.; 

 section 482; X 27 diam.), illustrating the stage when vesicles become concen- 

 trated in the ectoderm of the third and fourth arches (cf. with fig. 5). Note the 

 thickened epithelium covering the mound on the third arch. 3, 4, 6, aortic arches. 



12 to 17 Consecutive serial sections of an embryo of 4 days and 3 hours (H. 

 E. C, Ser. 1943; 12.0 mm.; sections 215 to 220 respectively, X 27 diam.). The 

 first three, on the left, show the mound which forms on the third arch and its 

 mesodermal core; the last three, on the right, show the thickened epithelium at 

 the lower end of the mound, out of which the first filaments are differentiating. 



12 7, II, III, ectodermal grooves; II, III, IV, pharyngeal diverticula. 

 15 E-III, E-IV, evaginations of the ectodermal epithelium on the third and 

 fourth arches, respectiveh'. 



230 



