THYMUS IN" THE MARSUPIALS. SJil 



medullary portions was very evident. There was no cervical 

 portion as distinct from a thoracic. Paired aggregations of 

 lymplioidal tissue lay without the thorax, external to the pec- 

 toralis muscles, and similar small nodules lay in contact with 

 the posterior borders of the submaxillary glands. I am dis- 

 inclined to regard these as representing detached portions of the 

 thymus. 



AntecJiinomys lanigera. 



There was a thoracic thymus consisting of two paired lobes in 

 the usual place, on the base aad ventral aspect of the heart. In 

 the neck I could find no thymus. Wedged in between the sub- 

 maxillary and parotid glands and at their anterior borders there 

 was a very small separate lobe, which on microscopic examina- 

 tion exhibited rather the characters of a lymph-gland than of a 

 true thymus, so that I am disposed to regard it as belougiug to 

 the former class of organs. 



III. Peramelid^. 



Perameles Gunni. Adult specimen, full-grown. 



The thymus (PL 37. fig. 3) was well developed and entirely 

 thoracic. It consisted of two lobes (tm.), extendiag from a little 

 below the first rib to about halfway between the second and third. 

 Opposite the first rib, where the peiicardial sac was narrow, 

 the thymus was elongated dorso-veutrally and somewhat massive. 

 Further down it thinned out. The right lobe was thicker than 

 the left, which was very thin and closely applied to the peri- 

 cardium. There was no external sign of division into secondary 

 lobules. There were no surrounding fat-masses, and except for 

 a slight irruption of fatty tissue the microscopic structure was 

 quite normal. 



Perameles Gunni. Foetus : head-length 1'7.5 centim. (PL 38. 

 fig. 9.) 



There was no cervical thymus present. The thoracic organ 

 {tm.) was relatively larger in this specimen than in any other 

 foetus I have examined. It began at about the anterior limit of 

 the sternum as two massive lobes with much secondary lobulation. 

 At its point of greatest development it measured two-ninths of 

 the dorso-ventral diameter of the thorax at that plane, and two- 



