GILL-FILAMENTS IN SAUROPSIDA 207 



nection with the fact that the vesicles first appear in the entoder- 

 mal walls of the gill clefts, is the observation, based on the work 

 of Greil and others, that it is the entoderm of the pouches of 

 frogs and toads, which initiates the process of gill formation. 

 This it does by spreading out from the distal ends of the pharyn- 

 geal pouches on either side of a cleft until the ectoderm that 

 covers the arches has become partly, and in some species com- 

 pletely, underlaid by a new layer of entoderm which has inter- 

 polated itself bet-ween the ectoderm covering the arches and the 

 mesenchyma upon which the ectoderm formerly rested. 



Following the first appearance of cysts in the chick, scattered 

 vesicles may arise in the walls of all the pharyngeal pouches and 

 in the ectoderm between the clefts on the outside. Toward the 

 end of the fourth day they become most numerous in the ecto- 

 derm between the second and third clefts, where the degenera- 

 tion is so rapid that pycnotic nuclei may appear anywhere in 

 the epithelium without grouping themselves into vesicles and in 

 sufficient numbers to give the epithelium a punctate appearance. 

 Eventually most of them are crowded down into the underlying 

 tissues. A second, less conspicuous concentration area occurs in 

 the ectoderm between the third and fourth clefts. In frogs and 

 toads it is the ectoderm of these two arches which evaginates to 

 form the first external gills. 



Before describing the further development of the epithelium, 

 it is desirable to review the changes in the branchial arches as 

 seen from the outside. Figure 4 represents these arches at the 

 stage when the epithelial vesicles first appear. It will be noted 

 that the hyoid arch is already larger than the others, that the 

 third arch has become somewhat wedge-shaped and smaller, and 

 that on account of this differentiation the branchial region as a 

 whole has lost that regularit}^ of arches and clefts so characteristic 

 of early stages in all vertebrate embryos. In the next figure these 

 differences are still further accentuated, the three-cornered hyoid 

 arch dominating the whole region, while the posterior arches are 

 depressed into a sinus below the general level of the neck and 

 are correspondingly reduced. In certain fishes and amphibians 

 it is the posterior margin of this second arch, now known as an 



