546 MR. JAMES JOHNSTONE ON THE 



posterior lobes (PL 38. fig. 8, tm.p.) are similar in structure to 

 those in M. Eugenii. 



The Cervical Thymus. (PI. 38. fig. 7, tm.c.) 

 Symington (1) has described this structure in several forms 

 belonging to the Macropodidae, and gives a photograph in his 

 paper showing its relationships to the other organs in the neck. 

 As the specimen from which his section was taken was a much 

 older animal than the foetuses I have examined, I propose to 

 indicate some features of interest exhibited in my sections. 



This organ was present in all the specimens I examined. 

 Its structure was that of a thymus. In the more median 

 sections a differentiation into cortical and medullary regions 

 was evident. Secondary lobulation had not progressed to any 

 considerable extent, although septa of connective tissue could 

 easily be traced into the interior of the body. There were 

 indications of the formation of a capsule. Comparatively few 

 blood-vessels were present in the septa. In the portion corre- 

 sponding to the cortex of a fully-developed thymus the cells 

 were small, closely compressed, and darkly staining; in the central 

 portion they were more sparsely scattered and less darkly 

 staining. 



In M. Eugenii, from the anterior extremity of the cervical 

 thymus a cord of cells (fig. 7, c.c.) could be followed upward 

 towards the head. This cord was surrounded by loose connective 

 tissue continuous with, and in fact expanding over, the connective 

 tissue surrounding the gland. It was not a continuous string 

 of cells, but consisted rather of detached elements ; there was, 

 however, a continuous tract of rather dense connective tissue, 

 in which these lay. Occasionally these tracts of cells could 

 be seen to be aggregated together into follicular structures, 

 each consisting of a group of little darkly staining cells sur- 

 rounding a more or less well-defined space. These cells stained 

 less darkly with carmine than those composing the thymus. 

 Indeed the portions of which this string was composed resembled 

 nothing so much in histological structure as the thyroid itself 

 at this stage, and the whole thing might fitly be compared 

 to a thyroid body of the stage of development represented 

 by the specimen, ravelled out to form an elongated series 

 of imperfect follicular masses more or less connected with 

 each other. At the anterior extremity of the cervical thymus 



