THE MAMMALIAN FIFTH ARCH 503 



In the rabbit and the pig the glandule thyroidienne is generally 

 considered anterior to the pre-pulmonic caecum. 



The posterior evagination in the cat is figured by Coulter as 

 having dorsal and ventral portions, the dorgal portion having be- 

 come constricted into anterior and posterior divisions. The ante- 

 rior division is more directly continuous with the ventral portion, 

 and is labeled the dorsal diverticulum of the fourth pouch. The 

 posterior division is represented as the fifth pouch. If the entire 

 dorsal portion of the evagination be considered anterior, as in 

 other forms, the fifth pouch of the cat would be morphologically 

 anterior to the ventral diverticulum of the fourth pouch. If as 

 stated by Verdun (and as I have observed in a limited study 

 of the development of the evagination in the pig) the earliest 

 indication of the Y-shaped pouch is its ventral portion, Coulter 

 erred when he labeled the most caudal pharyngeal prominence of 

 his 4.5 mm. cat as 'pouches 4-5.' 



The 'glandule thyroidienne,' like the pouches anterior to it, 

 comes in contact with the ectodern, differing radically in this 

 respect from the prepulmonic caecum. Occasionally I have found 

 it constricted into dorsal and ventral portions. In one instance 

 (fig. 16) which is very unusual, the glandule thyroidienne is 

 divided into dorsal and ventral portions, each of which has direct 

 connection with the exterior through the pre-cervical sinus. 

 Between the small diverging tubes thus formed, is the mesodermal 

 connection presumably between the fourth and fifth visceral 

 arches. The plane passing through the pre-pulmonic caecum 

 is shown slightly above (posterior to) that of the glandule thy- 

 roidienne. The occasional division of the glandule thyroidienne 

 into dorsal and ventral portions is suggestive of its representing 

 an independent pouch. 



Attention may profitably be called to the chronological rela- 

 tions of the portions of the double evagination to the associated 

 aortic arches, since these relations have hitherto remained un- 

 noticed. The 'glandule thyroidienne' and the 'fifth' arch, 

 despite their anterior position, appear respectively later than the 

 pre-pulmonic caecum and the pulmonic arch. (The same condi- 

 tions are found in birds.) 



