THYMUS IN THE MAESTJPIALS. 549 



nodules of tissue, presenting all the characters of a thymus. 

 These were embedded in an adipose matrix. Evidently fatty 

 degeneration had gone on to a marked extent. 



VII. General Considerations. 



"With one exception (the Koala) I have found the thymus 

 present in all Marsupials I have examined. It is diiEcult to say 

 much with regard to variation in size, since the time at which 

 the organ begins to uudergo fatty degeneration is probably very 

 different in different forms. Thus in a full-grown Didelphys 

 pusilla I found a very large thymus, with few indications of 

 fatty degeneration. In two specimens of TricJiosurus, both in 

 bad and ill-nourished condition, the thymus, on the other hand, 

 was reduced to a mere vestige, dilBcult to recognize, and very 

 likely to be passed over in the course of general dissection. It 

 is nearly always that part of the gland lying on the roots of 

 the carotids which persists, although, as various authors have 

 pointed out, the shape of the portion which lies over the base of 

 the heart can usually be recognized in the fat-mass to which it has 

 been reduced. This is not always the case, and some variation 

 in shape, too, exists. As a rule in the Marsupialia the thymus 

 consists of several lobes united in a common connective-tissue 

 investment, not necessarily symmetrical in relation to the median 

 body-line. In the young TricJiosurus I dissected there were two 

 lobes which were roughly lateral, paired, and anterior, and one 

 lobe which was unpaired, approximately median, and posterior, 

 and this disposition seems to be the usual one. In Thylacinus 

 there were two lateral and paired lobes, an anterior fat-body 

 which probably represents a former mass of thymus-tissue anterior 

 to the paired lobes, several approximately median masses, and an 

 asymmetrical and expanded mass lying on the upper half of the 

 heart. On the other hand, in Guscus, a form nearly allied to 

 TricJiosurus, I found one undivided median mass lying on the 

 base of the heart, and showing short paired and anteriorly 

 pointed prolongations passing upwards some little distance along 

 the course of the carotids. In some cases it would seem as if 

 such cervical prolongations of the thoracic mass of the thymus 

 had existed ; but in the retrogressive development of the organ 

 these have been usually the first parts to suffer degeneration. 

 Generally, as in many other mammals, the thymus extends down- 



LINN. JOUKN. ZOOLO&T, VOL. XXVI. 39 



