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The Development of the Thymus, Epithelial Bodies, and Thyroid 

 in the Marsupialia. Part II. — Phascolarctos, Phascolomys 

 and Perameles. 



By Elizabeth A. Eraser, B.Sc. 



(Communicated by Prof. J. P. Hill, F.R.S. Received March 10, 1915.) 



(Abstract.) 



This communication is a further contribution to our knowledge of the origin 

 and development of the thymus and thyroid glands in the Marsupialia and 

 contains a short account of these glands in two of the Diprotodontia, viz. 

 Phascolarctos and Phascolomys, and in one of the Polyprotodontia, viz. 

 Perameles. 



Phascolarctos and Phascolomys. 



Phascolarctos and Phascolomys normally possess only the paired superficial 

 cervical thymus gland ; only in one foetus of Phascolarctos out of nine 

 examined was a representative of the paired thoracic thymus (thymus III) 

 found. One pair of epithelial bodies, viz. epithelial body III, is always present, 

 whilst a second pair, presumably epithelial body IV, although found in the 

 wombat, may not always occur in the koala. 



As in Trichosurus, the cervical thymus is mainly of ectodermal origin but 

 entoderm to some extent takes part in its formation. In Phascolarctos, it 

 arises from the coalesced walls of the dorsal portion of the cervical sinus 

 opposite the third gill-pouch and extending down opposite to the connection 

 of the fourth pouch with the sinus, the entodermal component being probably 

 derived from both ductus ecto-entobranchialis III and IV. The third and 

 fourth pouches have the same form as in Trichosurus but the fusion of 

 ectoderm and entoderm, which constitutes the ductus ecto-entobranchialis III 

 and IV, becomes very thick and solid, especially in the case of the ductus ecto- 

 entobranchialis III, which passes into direct continuity with the primordium 

 of the cervical thymus. Although the second pouch is long and narrow and 

 an elongated ductus ecto-entobranchialis II is formed, the latter lies anterior 

 to, and has no connection with, the thymus primordium. In Phascolomys, on 

 the other hand, the primordium is derived from the ectoderm of the closed 

 cranio-dorsal walls of the cervical sinus together with a certain amount of 

 entoderm from the ductus ecto-entobranchialis II as in Trichosurus. 



In Phascolomys, the whole of the third pouch apparently gives rise to 

 epithelial body III and this is probably also true in the case of Phascolarctos 



