97 



The Development of the TJiymus, Epithelial Bodies, and Thyroid 

 in the Marsupialia. Part I. — Trichosurus vulpecula. 

 By Elizabeth A. Teaser, B.Sc. (Lond.), and Prof. J. P. Hill, F.E.S. 



(Received March 10, 1915.) 



(Abstract.) 



In recent years much attention has been paid to the development of the 

 thymus and thyroid glands in the higher Mammalia but no observations 

 on the development of these structures are extant in the case of the 

 Marsupialia. In this memoir the authors have attempted to fill this blank 

 in our knowledge so far as the Diprotodont Trichosurus is concerned. They 

 have had at their disposal an extensive material of that form comprising both 

 uterine and fcetal specimens, grouped in 22 stages. 



In the adult, the thymus is remarkable in that it consists of three pairs of 

 glands, viz., a large paired superficial cervical thymus situated posteriorly to 

 the submaxillary salivary glands and internally to the platysma on the 

 ventral side of the anterior region of the neck, and two pairs of smaller 

 thoracic or posterior cervical glands, situated the one behind the other, 

 cranially to the pericardium and in relation to the corresponding common 

 carotid arteries. These latter glands represent respectively paired thymus III 

 and IV. They may remain separate or the two glands of the same side may 

 unite with each other on one or both sides of the body. 



The epithelial bodies (parathyroids so-called) comprise two pairs which are 

 constantly present, viz., the primary epithelial bodies III and IV and in 

 addition a variable number of accessory bodies, not necessarily paired, which 

 are frequently found in connection both with the cervical and thoracic 

 thymus glands. Epithelial body III lies adjacent to the fork of the common 

 carotid artery, whilst epithelial body TV is usually situated in the proximity 

 of thymus IV. 



The thyroid is situated just caudally to the larynx and consists of two 

 lateral lobes connected by a median bridge. 



As concerns the development of the thymus, our observations show that the 

 epithelial basis of the cervical gland is derived mainly at least from the 

 ectodermal walls of the cranial portion of the closed-off cervical sinus, and to 

 a smaller extent from the distal portion of the ductus ecto-entobranchialis II. 

 The second gill-pouch in early stages is well developed and possesses an 

 extensive area of fusion (closing membrane) with the second ectodermal 

 groove. As development proceeds, however, the pouch becomes drawn out to 



