I. The development of the thymus, epithelial bodies, and thyroid in the marsupialia. Part I.— Trichosurus vulpecula

Abstract

In recent years much attention has been given to the development of the thymus and thyroid in the Mammalia, with the result that we are now in possession of more or less adequate accounts of the developmental history of these structures in representatives of many of the mammalian orders, including the Monotremata (Maurer, 34). As concerns the Marsupialia, however, apart from a brief résumé of results communicated by the present authors to Section D at the Dundee (1912) Meeting of the British Association, no observations on the origin of these structures are extant. It is the object of the present paper to fill this blank in our knowledge so far as the Diprotodont Trichosurus is concerned. In Part II, one of us (E. A. F.) gives an account of the development of these glands in certain other Marsupials of which our material is less extensive than it is in Trichosurus. Our work in this field is the outcome of an endeavour to prepare a normal table of the development of Trichosurus, but we had not proceeded very far with this task before it became evident that the work of tabulation of the conditions in the pharyngeal region could only be satisfactorily effected after we had made as thorough a study as possible of the development of the gill-pouches and their subsequent transformation. The results of that study are presented in this communication. They are, we venture to think, of considerable interest, inasmuch as they show that Trichosurus, in addition to being provided with a paired superficial cervical thymus, the epithelial basis of which is mainly of ectodermal origin, possesses two pairs of thoracic thymus glands derived exclusively, so far as their epithelial basis is concerned, from the entoderm of the third and fourth pairs of gill-pouches. In respect of the invariable development of these two thymus-metameres, Trichosurus exhibits more primitive relations than any of the mammals hitherto investigated. Further, in respect of the mode of origin of thymus III from the entire caudal as well as the ventral wall of gill-pouch III, Trichosurus would seem to furnish an example of the transitional stage between the Reptilian mode of thymus-development (the thymus being an exclusively dorsal product of the gill-pouch entoderm) and the Eutherian mode (the thymus arising as a ventral product of the gill-pouch).