222 EDWARD A. BOYDEN 



modern and highly specialized group; and that the degree to 

 which retrograding structures are developed does not necessarily 

 correspond to the rank which the possessor of these structures 

 holds in a graded phylogenetic series. The fact that these struc- 

 tures are present at all in reptilian embryos greatly increases the 

 significance of the better developed filaments in the chick embryo. 

 In discussing the conditions in reptiles it should be borne in 

 mind that considerably less material was available than in the 

 study of the chicks where some seventy embryos between the 

 sixth and ninth days of incubation were examined. Of the four 

 reptile series in the Harvard Collection, which are sufficiently 

 extensive to afford a fairly complete picture of the development 

 of the branchial region, three of them, Lacerta, Eutaenia and 

 Chrysemys, show epithelial outgrowths behind the hyoid arch 

 which are identical with the filamentous structures found in the 

 chick. The first of these, Lacerta muralis, presents a more primi- 

 tive branchial system than is found in birds, five well-spaced ecto- 

 dermal grooves being visible from the outside at an early period. 

 Later in its development the operculum fuses with the region 

 behind the fourth arch in such a way as to form a peribranchial 

 chamber into which portions of the third and fourth arches with 

 their respective aortic trunks freely protrude (H. E-. C, Ser. 813; 

 6.4 mm.). Still later, when the branchial chamber has become 

 obliterated, small epithelial proliferations appear from under- 

 neath the operculum in the region of its fusion with the posterior 

 gill arches (H. E. C, Ser. 811 and 812; 7.4 mm.). Although 

 they have but a transitory existence they occur at the same rela- 

 tive time and place as the filaments in the chick, with the dif- 

 ference that the filaments in the birds appear on the third and 

 fourth arches prior to their fusion with the operculum as well as 

 afterwards. In Aristelliger praesignis this is apparently re- 

 versed; the epithelium of the third arch is very much thickened 

 just prior to fusion with the operculum, but thereafter no fila- 

 ments are to be observed, as if a somewhat premature fusion, as 

 compared with conditions in other embryos, had inhibited the 

 epithelial proliferation which had already started (H. E. C, Ser. 



