DEVELOPMENT OF SUPRAPERICARDIAL BODY 373 



pharynx and form small isolated cysts. Later, the surrounding 

 connective tissue invades the organ, dividing it into two or three 

 small lobules. In the adult these bodies come to lie so near 

 the thyreoid that they are considered, by De Meuron, as acces- 

 sory thyreoids. 



In Lacerta a diverticulum of the ventral pharynx-wall arises 

 behind the fourth gill-pouch on either side. The diverticulum 

 on the right side atrophies very early, while that on the left 

 persists. De Meuron calls this body the accessory thyreoid, and 

 believes it is homologous with the suprapericardial body of 

 selachians and amphibians. 



In birds and mammals De Meuron describes symmetrical 

 diverticula arising from the ventral extremities of the fourth 

 pair of gill-pouches. He considers these diverticula as homolo- 

 gous with the accessory thyreoids of reptiles and the supraperi- 

 cardial body of selachians and amphibians. 



Maurer ('87), studying Amphibia, describes the suprapericardial 

 bodies and calls them postbranchial bodies because of their posi- 

 tion behind the last branchial pouch. In Anura they arise 

 symmetrically from the ventral pharynx-wall behind the fifth 

 branchial pouches. Later the bodies become separated from 

 the pharynx and form, either a single large follicle or a complex 

 of smaller ones. The follicles never develop colloid, but almost 

 always, in some phase of their development, contain a serous 

 secretion. 



In urodeles the postbranchial body develops only on the left 

 side. It arises as a solid epithelial bud which early acquires a 

 small central lumen. This strand separates from the pharynx, 

 becomes placed obliquely to the long axis of the pharynx and 

 in some cases buds off a few smaller epithelial masses. Maurer 

 did not find the postbranchial bodies in teleosts. 



According to Maurer's description, the postbranchial bodies 

 in amphibians in no way resemble the thyreoid and can not be 

 called accessory thyreoids. They occasionally show a serous 

 secretion but never develop colloid. Maurer believes that De 

 Meuron described correctly the origin of the postbranchial 

 bodies, but that he has confused their later development with 



