GILL-FILAMENTS IN SAUROPSIDA 221 



In figure 9 the primary and secondary series have maintained 

 their identity to the end. In reviewing the origin of both series 

 it will be seen that the posterior wall of the hyoid arch is poten- 

 tially a filament-bearing surface as well as the walls of the third 

 and fourth branchial arohfes. This is in accord with what we 

 know of conditions in gill-bearing vertebrates. 



By way of a summary the history of these vestigial gill fila- 

 ments in the chick may be divided into six stages: 1, the appear- 

 ance of degeneration vesicles in the branchial epithelium; 2, the 

 concentration of these in the ectoderm covering the third arch 

 and, to a lesser extent that covering the fourth arch; 3, the thick- 

 ening of the ectoderm of these two vesiculated areas into tufted 

 epithelial mounds, and, in the case of the third, an apparent 

 evagination of the ectoderm with, a mesodermal core; 4, a gradual 

 differentiation of these areas (now crowded into one and fused 

 with the sides of the backward growing operculum) into a trans- 

 verse line of filaments on each side of the neck; 5, a progressively 

 medial displacement of this line, correlated with the medial mi- 

 gration of structures in the pectoral body-wall; 6, a rather rapid 

 reduction of this line and the eventual suppression of both fila- 

 ments and operculum. 



GILL FILAMENTS IN REPTILES 



Although the branchial reg'.on of reptilian embryos exhibits 

 some measure of transition between the higher amniotes and the 

 gill-bearing vertebrates it is much more nearly akin to that of 

 birds than to that of any other group. Particularly is this true 

 with regard to the development of the hyoid arch, where in tur- 

 tles, lizards, and alligators (as in birds) the opercular processes 

 of the two sides unite to form a conspicuous fold of tissue a/)ross 

 the ventral surface of the neck which persists long after all trace 

 of the other gill arches has disappeared. Similarly, vestiges of 

 filamentous structures behind the hyoid arch are to be found in 

 at least three of the main groups of reptiles, although in a more 

 transitory and less conspicuous form than obtains in the chick. 

 That they are not developed to a higher degree in the former may 

 be explained by the fact that living reptiles are themselves a 



