PHARYNGEAL DERIVATIVES OF AMBLYSTOMA 657 



sjrmmetrical in development, arising from either side of the 

 pharynx, as a small diverticulum which soon forms isolated 

 cysts lying on either side of the trachea, above the pericardium. 

 In the adult, the cyst lying on either side was close to the thyreoid 

 and was thus considered by de Meuron as 'accessory thyreoid,' 

 although he was unable to follow the details of development. 



Maurer ('88), was the first to trace the development of the body 

 in both Anura and Urodela, and, because of its developmental 

 position with reference to the branchial pouches, he gave it the 

 name ' postbranchial body.' In the anurans (Rana and B^fo), 

 the body develops symmetrically behind the fifth pouch, just 

 lateral to the aditus laryngeus muscle, and consists of either single 

 or complex follicles, which never contain colloid. They retain 

 their early position close to the thyreoid, but, according to 

 ]\Iaurer, never unite with that gland in anurans. In the urodeles 

 (Triton and Siredon), he found the body arising as a solid epi- 

 thelial bud from the floor of the pharynx on the left side; this, 

 after elongating, becomes separated from the pharynx, and takes 

 a direction parallel to the long axis of the larval body. Late in 

 development it acquires a lumen which never contains colloid. 

 It does not in any way, according to Maurer, resemble the thy- 

 reoid, and therefore in his view can not be considered as an 

 'accessory thyreoid' as was de Meuron's belief. Maurer be- 

 lieved de Meuron described correctly the origin of the post- 

 branchial body in Anura, but thought he confused its later devel- 

 opment with the epithelial derivatives of the pouches, which later 

 come to lie close to the adult thyreoid gland. Maurer agrees 

 with de Meuron that the structure in Anura is homologous with 

 that of the selachians. The unilateral position of the body in the 

 urodeles, which he studied, and the persistance of the connecting 

 stalk with the pharynx, suggests to him that perhaps it represents 

 a remnant of the ductus oesophago-cutaneus of Bdellostoma. 



Miss Piatt ('96), found that the body developed symmetrically 

 on either side in Necturus, and thus pointed out that Maurer' s 

 conception of the asymmetrical development in the urodeles as a 

 whole to be erroneous. In addition to this she found tnat tne 

 structure arises as a small vesicle (in 15 mm. larva), from the ven- 



