508 



a complete arch. . . There are not in the rabbits observed, two dis- 

 tinct entodermal pouches in this region, as exist in pig embryos." 

 The fifth aortic arch in the pig is described and figured as a vessel 

 leaving the fourth arch to enter the dorsal aorta. It has a branch 

 passing into the pulmonary arch, and is associated with other irregular 

 vessels. It is, however, in typical relations with the pharyngeal pouches. 



Reviewing all these observations, it appears that only in the 

 rabbit and sheep, as recorded by Zimmermann, is a typical fifth arch 

 found among mammals; and that his observations in the rabbit have 

 not been confirmed by the writer, or by Miss Lehmann. 



In regard to the pharyngeal pouches it is to be noted that Boas' 

 prediction and Zimmermann's observations call for five pairs of pouches 

 with the sixth arches behind the last. At a certain stage of develop- 

 ment the fourth pouch is commonly described as a >h - shaped body, 

 of which the upper or anterior arm extends to the ectoderm, forming 

 the true fourth pouch. The lower or posterior arm, generally called 

 the postbranchial body, does not approach the ectoderm. It is an 

 open question whether it is an appendage of the fourth pouch, a 

 rudimentary fifth pouch, or a structure essentially different from the 

 branchial pouches. Verdun ^) describes its development accurately as 

 follows: — "Although the postbranchial body appears in the highest 

 forms as a coecum appended to the fourth branchial pouch, the study 

 of the youngest stages shows that it is in reality a diverticulum proceed- 

 ing directly from the pharynx immediately behind and internal to the last 

 cleft (the fourth pouch). . . The two formations primitively distinct 

 and simply contiguous, grow in such a way that later they open into 

 the pharynx by a common opening.'" This is true of the rabbit and 

 pig as will be shown later. The postbranchial body subsequently 

 grows like a simple tubular gland to meet the median thyreoid. It 

 becomes detached from the pharynx and its end proliferates. Born-) 

 thought that it became an integral portion of the thyreoid. Verdun ■^) 

 considered that in the rabbit it disappeared entirely or formed cystic 

 central cavities in the thyreoid lobes. Maurer'^) finds it significant 

 that the postbranchial bodies of Echidna, though they do not unite 

 with the median thyreoid, produce a colloid secretion. The fate of 

 the fourth pouch is even more obscure. It loses its connections with 



■ ■ ! 



1) P. Verdun, Derives branchiaux, Toulouse 1898, p. 178. 



2) G. Born, Arch. f. mikr. Anat., Bd. 22, 1883, p. 304. 



3) P. Verdun, 1. c. p. 141. 



4) P. Maurer, Hertwig's Handbuch der Entwickelungslehre, Bd. 2, 

 Abt. 1 1902, p. 146. 



