THYMUS IN- THE MARSUPIALS. 5i5 



ispace surrounded it, due evidently to a contraction of the 

 cell-mass. Capillaries were present in the tissue immediately 

 surrounding tlie body, but none entered its substance. There 

 ■was, of course, no distinction as yet into cortical and medullary 

 parts, but septa from the surrounding connective tissue had 

 begun to penetrate into the general cell-mass, whereby incipient 

 lobulation may be said to have commenced. 



The superior lobe (tm.a.) differed completely in histological 

 structure from that just described. Considering it apart from 

 its relations to surrounding tissues, I should have had no hesita- 

 tion in describing it as a thyroid, or part of a thyroid mass, 

 judging from its minute structure. It was composed of twisting 

 strings of small cells, with occasional anastomoses. The lumina 

 of these strings were not very evident, but my sections were 

 thick enough to allow of one of them lying uncut, and in 

 optical section the sparseness of the cells in its interior could 

 be easily seen. Where cut in transverse section, they could 

 be seen to be tubes, with contents consisting of scattered cells 

 with indefinite outlines filling up their lumen. The capsule was 

 no more obvious than in the case of the posterior lobe ; but the 

 usual annular space surrounded the body, across which sheets 

 of connective tissue could with ease be followed into the in- 

 terior, ramifying among tRe cell-strands. There were few blood- 

 capillaries either round or in this body. 



I have seen much the same thing in transverse sections of 

 MacropusWzIcoxii, and in a series of sections shown me by Mr. M. 

 F. "Woodward, which were probably cut from a Macropus. In 

 M. Wilcoxii there are some differences. The superior lobe of the 

 thymus (PI. 39. figs. 11, 13, tm.a., & 12) is by no means so sharply 

 delimited from surrounding parts as in the foetus described 

 above. Por the most part it lies superior to the other lobe of 

 the thoracic thymus, but portions of it are found lateral and 

 even posterior to the latter (PL 39. fig. 11, tm.a.). Detached 

 portions of its darkly staining tissue are to be found along 

 the neck for some distance, lying in the tissue surrounding vagi, 

 sympathetics, and internal jugulars. Little nodules lie in the 

 substance of the inferior lobe itself (PI. 39. fig. 11, tm.a?), round 

 its periphery, and always in little spaces sharply limited by 

 single sheets of connective tissue. Sometimes these are seen in 

 transverse section to be really little follicles enclosing spaces and 

 composed of few cells. Sometimes they are solid cords. The 



