Development of Thymus, etc., in Marsupialia. 99 



near the bifurcation of the common carotid artery, whilst thymus III moves 

 backwards. 



The fourth gill-pouch is smaller than the third, but undergoes a corre- 

 sponding development. Here, however, epithelial body IV is derived from 

 the dorsal portion of the pouch, including both its cranial and caudal walls, 

 whilst thymus IV takes origin from its ventral portion, including its solid 

 ventral prolongation. 



Trichosurus is the first mammal to be described in which a fully-developed 

 thymus derived from the fourth gill-pouch is constantly present. Eudiments 

 of a thymus IV have, however, been observed in a number of Eutheria 

 (calf, cat, man, etc.), whilst thymus IV is regularly present in some reptiles, 

 e.g., Coluber and Tropidonotus. In respect of the constant presence of 

 thymus IV, Trichosurus would appear to exhibit more primitive relations 

 than any mammal hitherto investigated, whilst in respect of the mode of 

 origin of thymus III, from the whole extent of the caudal wall of the pouch 

 as well as from the ventral prolongation, Trichosurus would seem to furnish 

 an example of the transitional stage between the Eeptilian mode of thymus 

 development (the thymus being an exclusively dorsal product of the gill- 

 pouch) and the Eutherian mode (the thymus arising as a ventral product of 

 the pouch). 



The developmental history of the median thyroid primordium in Tricho- 

 surus does not differ essentially from that of other mammals. It gives 

 origin without doubt to the main mass of the adult thyroid. It is probable, 

 however, that the ultimo-branchial (post-branchial) bodies also contribute in 

 some degree to the formation of the lateral thyroid lobes. The ultimo- 

 branchial body appears in our earliest stage as a ventral prolongation of the 

 small fifth gill-pouch. It very soon attains a considerable size and, after 

 separating from the pharynx, it moves forwards and becomes closely con- 

 nected with the dorso-medial surface of the corresponding lateral lobe of the 

 thyroid. It then proliferates actively and gives off cellular sprouts which 

 penetrate amongst, and become indistinguishable from, the cellular cords of 

 the lateral lobe. 



