554 MR. JAMES JOHNSTONE ON THE 



from the median anlage, and that the lateral anlagen always 

 remain very unimportant organs in respect of size, and never inti- 

 mately fuse with the main body of the gland. Nevertheless these 

 lateral thyroids are in a way comparable to the median thyroids 

 as is indicated by the persistence for a certain period of the 

 hollow peduncle uniting them with the original branchial clefts 

 — the " canal thyreo-pharyugien." 



The developmental history of the internal and external epithelial 

 bodies of Kohn was worked out by Simon, and the insufficiency 

 of these terms pointed out. The internal epithelial body he 

 showed to be identical with the " glandule thyroi'dienne " and a 

 derivative of the 4th pouch. The external epithelial body, on the 

 other hand, is identical with the " glandule thymique " and takes 

 its origin from the 3rd — that which gives rise to the thymus. 

 In addition to these epithelial bodies, the lateral lobes of the 

 thyroid enclose nodules of tissue of lymphoid, or thymic nature. 

 The genesis of these bodies has been treated in a somewhat 

 unsatisfactory manner by Simon, who considered them to be 

 appendages of the thymus, but it is evident that their develop- 

 ment requires further investigation. 



Neither the embryology of the thyroid, and thymus, nor the 

 question of the presence or absence of epithelial bodies in the 

 former, has been worked out in the Marsupials. Judging from 

 the disposition of the organs in the young animal, it would seem 

 that the development of those structures might diifer in many 

 details, both from that in the other Mammalia outlined above and 

 indeed in the various families of Marsupials themselves. Thus 

 the presence of a cervical thymus and the association of this 

 body with the lateral lobe of the thyroid are points of difference. 

 For it is not probable, as Symington has indicated, that the cervical 

 thymus is developmentally the same structure as the cervical 

 prolongations of that organ found in some other mammals. 

 And the absence in the young marsupial of any structures 

 exactly resembling " epithelial corpuscles " offers another point 

 of dissimilarity. 



The cervical thymus may probably arise from the 4th 

 branchial pouch. This is suggested by its association with the 

 lateral lobe of the thyroid, assuming that the latter represents a 

 part of the thyroid developed from the 4th pouch as in the other 

 mammalia. This, however, is by no means certain. Then the cell- 



