PLATE 3 



EXPLANATIOX OF FIGURES 



Sections illustrating later development of filaments 



18 and 19 Sections through the hyoid region of an embryo of 5 days; 3.0 mm. 

 (H. E. C, Ser. 1951; sections 245 and 247 respectively) X 67 diam. 3, 4, 6, aortic 

 arches; //, second ectodermal groove; ///, diverticulum of third pouch; op., 

 operculum ;/i, filaments at the upper end of the 'wedge ;'/2. filaments at the point 

 of the 'wedge' (cf. with fig. 6). 



20 Frontal section through the opercular fold and pectoral wall of an ombrj-o 

 of 6 days and 2 hours; 16.0 mm. (H. E. C, Ser. 1950; section 901) X 67 diam. (cf. 

 with fig. 8). op., operculum;/., branched filaments; s. p., pectoral grooves; cm., 

 mesothelial ridge; h.c, bulbus cordis; p., pericardial cavity; c.p., thickened epi- 

 thelium which gives rise to the pectoral ridges. 



21 Filaments of a 10.0 mm. chick (H. E. C, Ser. 2038; 5 days; section 572) 

 X 67 diam. (cf. with fig. 22). S, 4, 6 aortic arches; III, IV, diverticulum of 

 third and fourth pouch; e., esophagus; //•., trachea; p., pericardial cavity. 



22 Filaments of a 10.0 mm. turtle embryo (H. E. C, Ser. 1078 Chrysemys 

 marginala, section 286) X 67 diam. 



232 



