GILL-FILAMENTS IN SAUROPSIDA 223 



1884; 4.9 mm.). The same is true of Sphenodon punctatum 

 (H. E. C, Ser. 1491; 7.9 mm.). 



In contrast with these Uzards the snake Eutaenia presents a 

 very interesting condition. So great is the lengthening process 

 to which the body as a whole is subjected that the gill arches and 

 clefts are obliterated by being drawn out instead of being crowded 

 together. There is no opportunity for the formation of a peri- 

 branchial chamber nor even for a ventro-medial union of the 

 two hyoid arches to form the plica opercularis, so characteristic 

 of the Sauropsida as a whole. Consquently each hyoid arch 

 is pulled back on the side of the trunk and there undergoes a 

 further development by itself, persisting long after the other 

 arches have lost their identity. Just before these disappaar 

 (Eutaenia sirtalis, H. E. C, Ser. 1349; 7.3 mm.; and E. radW, 

 Ser. 1350 7.4 mm.) the epithelium of the operculum (in the first 

 case from the under side, in the second from the outer side) gives 

 rise to a tuft of cells comparable in point of time and position 

 with the filaments of the chick. Again, however, these are rather 

 small and transitory appearances. To see structures in the rep- 

 tile, closely comparable to those in the chick it is necessary to 

 examine turtle embryos {Chrysemys, H. E. C, Ser. 1078; 10.0 

 mm.; and Ser. 1083; 11. (J mm.). The first of these (fig. 22) has 

 been placed beside a chick embryo of exactly the same size (fig. 

 21, H. E. C, Ser. 2038; 10.0 mm.), which happily was so sec- 

 tioned as to permit a very striking comparison of the two em- 

 bryos, even to such details as the aortic arches, cephalic veins, etc. 

 A glance at the operculum and its underlying filaments in the 

 two specimens shows that at least in the stage at hand we are 

 dealing with almost identical structures. Again, however, these 

 filaments have but an ephemeral existence as compared with the 

 development which the same structures undergo in the chick. 



DISCUSSION OF LITERATURE 



Of considerable interest in connection with this paper is the 

 exhaustive work of Ekman on the branchial region of the Anura. 

 He conducted a series of experiments to determine the \'arious 

 factors involved in the production o;" gill filaments in frog and 



