BRANCHIAL DERIVATIVES PIED-BILLED GREBE 27 



assume the characteristic structure of postbranchial bodies, con- 

 sisting essentially of compact epithelial cords and lobules, and a 

 varying number of globular vesicles with walls of cubical or 

 cylindrical epithelium. In the duck Verdun and Hamilton de- 

 scribe conditions virtually the same as in the chick. In the 

 sparrow, on the other hand, Helgesson found that the post- 

 branchial body of the right side atrophies, having entirely dis- 

 appeared in embryos of 14 mm. length. That of the left side 

 persists, and was observed in full-grown individuals. 



THE BRANCHIAL DERIVATIVES IN THE GREBE 



In grebe embryos of about six days the third gill cleft has been 

 shut off from the exterior by the closing of the ectobranchial 

 duct III (terminolog}^ of Hammar, '13); its pharyngeal passage 

 or the entobranchial duct III may be entirely or only partly oc- 

 cluded. The fourth visceral pouch extends to the ectoderm, with 

 which its lateral wall is in intimate contact. The entobranchial 

 duct IV, which is still open, enters the pharynx, caudal to and 

 entirely independently of the entobranchial duct III, and that 

 of the left side bears laterally a well-developed spherical vesicle, 

 the postbranchial body. On the right side this body is less 

 clearly differentiated. Both third and fourth gill pouches are 

 situated somewhat obliquely and are flattened under the influence 

 of the aortic arches so that they present posterodorsal and 

 antero-ventral surfaces. 



At six and one-half days the changes in these two pouches con- 

 sist chiefly in their having become somewhat longer, with further 

 reduction of the ectobranchial and entobranchial ducts. Both 

 postbranchial bodies are well developed and have become some- 

 what fusiform, with thick walls and a narrow, but sharply defined 

 central canal. The right postbranchial body at this stage 

 reaches its maximum development. 



In embryos of about six and three-quarter days marked 

 changes are noticeable. The third visceral pouch has completely 

 separated from the pharynx and forms a V-shaped body whose 

 apex is directed laterally and is attached to the ectoderm by the 

 short solid ectobranchial duct. From the apex one limb of the 



