BRANCHIAL DERIVATIVES PIED-BILLED GREBE 35 



scrutiny of the sections, however, reveals on one side a shallow 

 but distinct groove on the outer surface of the diverticulum, 

 and on the opposite inner surface is a low ridge. This feature 

 can be traced through approximately the caudal half of the total 

 number of sections through which the diverticulum extends. 

 On the opposite side, although not apparent in the sections, a 

 tendency to division of the pouch is shown in a reconstruction 

 made to a magnification of 100 diameters. The lines of division 

 here indicated correspond to those separating the fifth and sixth 

 pouches in the five-day embryo and evidently denote the begin- 

 ning of this differentiation. The time of appearance of this divi- 

 sion well accords with Rabl's observations on the duck, where, 

 though not present in an embryo of four days and seven hours, 

 it was conspicuous in one ten hours older. 



The duration of the fifth visceral pouch is rather brief. In an 

 embryo of five and a half days (fig. 7) it already is more flat- 

 tened and is much less distinctly differentiated from the fourth 

 pouch, but it is still sharply marked off from the postbranchial 

 body which has increased in size and protrudes conspicuously. 

 By the sixth day the fifth pouch has been further reduced, so 

 that it is recognizable merely as a slight elevation in the dorsal 

 wall of the fourth visceral pouch, just lateral to the postbranchial 

 body. From the last named it is still distinctly demarcated, and 

 it clearly does not take part in the formation of this body. The 

 evidence from these stages indicates, therefore, that the fifth 

 pouch becomes separated from the postbranchial body along 

 with the fourth visceral pouch. Beyond this period I have not 

 been able to distinguish it and it is probable that, as suggested 

 by Rabl, it eventually becomes a part of the connecting stalk 

 which for a time unites the fourth pouch and the postbranchial 

 body. 



The degree to which the fifth and sixth pouches may become 

 •differentiated from each other in birds is possibly variable for 

 different groups. Where the fifth visceral arch is poorly devel- 

 oped the corresponding gill pouch must have more nearly the 

 appearance of a secondary lobe of the fourth, and the distinction 

 between the fifth and sixth, in turn, may then become more or 



