PHARYNGEAL DERIVATIVES OF AMBLYSTOMA 665 



The thymus bodies in the mammals are developed from the 

 ventral (rather than the dorsal) portion of the third and fourth 

 pouches, and this act throws considerable doubt on the homology 

 of these organs in mammalian and non-mammalian groups In 

 some cases also the ventral region of the second pouch (rabbit) 

 has been described as forming a thymus body. Although it is 

 beyond the scope of this paper to enter into a discussion of the 

 intricate points of horiiology, brief reference may be made to 

 the discussion by Kingsbury ('15), pp. 359-371. I quote his 

 conclusions. 



' 'Recognizing that the thymus-forming factors are not intrin- 

 sic, but extrinsic, i.e., partly a function of position and relation, 

 it is no longer necessary directly or completely to homologize 

 thymus bodies in different forms, since it is obvious that different 

 growth conditions may determine thymus development from 

 quite different portions of the branchial epithelium, and portions 

 that in one form may persist and undergo thymus transformation, 

 in others may degenerate and disappear without the character- 

 istic reaction appearing." 



' 'In closing .... it may be stated that there is a wide- 

 spread tendency to thymus-formation in the branchial region, 

 characterized by a persistence and growth of epithelium with a 

 characteristic (though not peculiar) reaction with adjacent tis- 

 sues, under conditions that are not yet fully analyzable. What 

 these conditions are and what determines the development of a 

 thymus or thymus bodies is unknown, and any attempt to de- 

 termine them awaits further analysis of growth conditions of the 

 region, particularly in the lower forms." 



Postbranchial body. This structure has not been described in 

 the cyclostomes. It appears in all elasmobranchs with the excep- 

 tion of Heptanchus, but is absent from all the teleosts yet studied. 

 In the other gnathostomes, its appearance and position is vari- 

 able, and its significance inthese forms has led to wide-spread 

 discussion, as is evidenced by the different names and inter- 

 pretations given it by various workers, as stated above. Its 

 phylogenetic significance is not clear. Some have suggested that 

 it is the representative of the ductus oesophago-cutaneus of 



