BRANCHIAL DERIVATIVES PIED-BILLED GREBE 33 



the fifth visceral pouch in birds. The diverticulum in question 

 later gives rise to the postbranchial body, which, therefore, in 

 Rabl's view, is not a derivative of the fifth pouch. 



Rabl's work seems to have been overlooked by Hamilton in his 

 study of the derivatives of the gill pouches in the duck, and his 

 attention apparently was not directed to a closer study of the 

 relationship of the postbranchial body to a possible sixth, as 

 well as to the so-called fifth visceral pouch. He does not allude 

 to similar observations in connection wath his specimens of 

 three and a half, five and a half, and six days, and he derives the 

 postbranchial body from what he refers to as the medial of two 

 globular parts into which the fourth visceral pouch has become 

 divided by a constriction about its middle portion. 



In grebe embryos of four and a half, five and five and a half 

 days I find conditions very similar to those observed by Rabl 

 in duck embryos of approximately corresponding ages. The 

 relations of the parts concerned are most clearly shown in the 

 five-day stage (fig. 13). On the caudo-medial wall of the fourth 

 visceral pouch are two well-defined diverticula, one medial, the 

 other lateral. The medial diverticulum is larger and deeper and 

 is situated at the junction of the fourth pouch with the pharyn- 

 geal wall. The area which the two diverticula together occupy 

 is clearly marked off from the fourth pouch itself, more so on the 

 left side, where, as previously noted, the postbranchial body 

 attains greater development than on the right. In the light 

 of later stages, the medial of the tw^o diverticula gives rise to the 

 postbranchial body and corresponds to the sixth pouch of Rabl; 

 the lateral one represents the fifth. The relations of these two 

 pouches to each other, to the fourth pouch, and to the pharynx 

 are essentially the same as those described by Rabl for duck 

 embryos of five days and sixteen hours and five days and eight 

 hours, represented by his text figures 5 and 6, respectively. 

 (That we are dealing with corresponding parts in the two species 

 will become further evident by referring to Rabl's work and 

 comparing his figures 4 e, 4 f, PL XI with figures 7 and 8 of the 

 grebe.) The mesenchymal ridge (V.a.VI) between the fifth and 

 sixth pouches is the sixth visceral arch of Rabl, and lateral to 



JOURNAL OF MORPHOLOGY, VOL. 31, NO. 1 



