THYMUS IN THE MARSUPIALS. 539 



along the sides towards the diaphragm, wliere they formed a 

 thick expanded mass. Similar tissue lay about the bases of the 

 carotids and great veins. In the midst of this fat, and extending 

 from a little above the first rib to about the third, were several 

 masses of tissue {tm.) presenting the characters of a thymus. 

 They were much elongated longitudinally. Two lay to the right 

 and one to the left, between the fii'st and second ribs. Two other 

 smaller masses lay approximately medially. All these portions 

 of thymus-tissue were flanked by the fat-masses, which met on 

 the diaphragm with a distinct median furrow between them. The 

 pericardium itself was also found to be fat-laden about the origin 

 of the great vessels. Microscopic examination showed the parts 

 I have termed thymus to possess the structure characteristic of 

 that organ. Fatty degeneration had, however, gone on to a very 

 great extent, and I suspect the gland must have extended much 

 further towards the apex of the heart at an earlier period. 



Dasyurus sp. Foetus ; length 7'5 millim. 



The head was strongly flexed, and the tissues were in a bad 

 state for detailed histological investigation. The thyroid was a 

 paired body with a well-marked isthmus and consisted of loosely- 

 packed, solid cords of cells. Its greatest length was about 0'5 

 miliim. It lay quite below the lower border of the cricoid 

 cartilage. 



The thymus was also a paired body, consisting on either side 

 of a smaller posterior and a larger anterior lobe. These lateral 

 portions at no point approximated closely. They presented the 

 usual characters of embryonic thymus-tissue, small densely- 

 packed cells, staining deeply with carmine. The anterior 

 portions began slightly above the first rib ; they were approxi- 

 mately O'di millim. in greatest length, and their greatest width 

 was only about 0*02 millim. They occupied the usual position, 

 almost touching the dorsal surface of the sternothyroid muscle 

 and lying between this and the carotids ; they were divided into 

 several unequal lobules. 



The posterior lobes were in all respects similar in histoJogical 

 structure to the anterior described above. They occupied a posi- 

 tion at about the transverse level of the top of the aortic arch. 

 They were situated somewhat wider apart than the anterior lobes, 

 and lay in little cavities as if they had retracted away from the 



