PHARYNGEAL DERIVATIVES OF AMBLYSTOMA 115 



thick) extending vertically downward from the floor of the 

 pharjaix. It soon elongates, loses its connection with the 

 pharynx (ll-13mm. larvae), and as an irregular, poorly defined 

 mass of -cells lies medial to the aditus laryngeus muscle. 



In one 19mia. larva, a postbranchial body was found on the 

 right side as well as on the left; the left, however, was the larger. 

 This is an unusual condition, since in none of the other speci- 

 mens studied did the structure on the right side occur, but is of 

 interest since it shows the possibility of some variability withia 

 a genus of urodeles. It is also important as bearing upon the 

 question of homologies of these structures. 



In the later lar\'al stages, the postbranchial body becomes very 

 irregular; in some regions it is solid, in other parts it shows fol- 

 licular structure, with cuboidal cells forming a fairly definite 

 layer enclosing a conspicuous lumen, but which in no case con- 

 tains colloidal material. 



During transformation, because of increase in the size of the 

 surrounding structures (especially the laryngeus and sterno- 

 hyoideus muscles), the postbranchial body is flattened laterally 

 and reduced to a sheet of poorly defined cells which now contain 

 no lumen. In the material studied by the writer the structure 

 and anatomical position of the postbranchial body in the adult 

 is variable. It could not be found by gross dissection. In sec- 

 tions of young adult heads, it is a fairly compact oval body com- 

 posed of regular cuboidal cells enclosing a spacious lumen. In 

 sections of old heads, the writer was not able to locate the struc- 

 ture. 



The carotid gland and epithelial bodies in Amblystoma begin 

 their development at the time of metamorphosis. There is no 

 evidence to show that the carotid gland is concerned in its gen- 

 esis with the epithelial cells of the degenerating gill-pouches. 



The anlagen of the epithelial bodies (two on either side) are 

 recognized as irregular longitudinal sheets of entodermal cells 

 (remnants of the ventral portions of the fourth and fifth vis- 

 ceral pouches) extending caudad (caudal limits poorly defined) 

 from points where the afferent branchial arteries enter their 

 corresponding gills in the early transforming stage. 



As transformation proceeds, the irregular longitudinal sheets 

 of entodermal cells become resolved into compact oval bodies 

 (two on a side) so that in the late transforming stage, these lie 

 one behind the other, close behind the carotid gland, each being 



