588 CHARLES EUGENE JOHNSON 



pouch and the uhmiobranchial diverticuUim in the tern it is 

 possible that in the chick the two diverticula are coalesced in 

 their early stages and only at a later period do they become dis- 

 tinguishable from each other, as the ultimobranchial body con- 

 tinues development while the fifth pouch becomes regressive. 

 On the other hand is the possibility that in a more extensive 

 series of chick embryos a condition similar to that in the tern 

 may be found. 



The next developmental stage at hand is represented by two 

 embryos of five and one-half days. In both of these the ulti- 

 mobranchial diverticulum is distinctly differentiated. Figure 4 

 is a wax reconstruction of the caudal pharyngeal diverticula of 

 one of the embryos, made at a magnification of 200 diameters. 

 The fourth visceral pouch was in close contact with the ectoderm. 

 On the posterior wall of this pouch, where it joins the pharjnix, 

 are two distinct outpouchings. The lateral and smaller one 

 clearly corresponds to the fifth pouch as it was represented in 

 the preceding birds; the larger medial one is the ultimobranchial 

 diverticulum. While the fifth pouch might here be interpreted 

 as a secondary evagination from the fourth pouch, the ultimo- 

 branchial diverticulum is more clearly an outgrowth from the 

 pharynx proper, and at this stage it opens into the pharyngeal 

 cavity separately from the common mouth of the fourth and 

 fifth pouches. The diverticulum as a whole is situated more 

 ventrally than the fifth pouch. As in the other l)irds investi- 

 gated, both the ultimobranchial diverticulum and the fifth pouch 

 are better developed on the left side than on the right. Text 

 figure 2 represents a transverse section through the middle of 

 the fifth pouch of this embryo. The section cuts the ultimo- 

 branchial diverticulum at its junction with the pharjmgeal wall, 

 leaving the greater part of it posterior to the plane of the section. 



In this embryo the fifth aortic arch could not be found on the 

 left side, but on the right it was clearly represented by a short 

 channel along the lateral side of the sixth arch, opposite the 

 shallow groove formed between the fourth and fifth visceral 

 pouches. 



