II. The development of the thymus, epithelial bodies, and thyroid in the marsupialia. Part II.—Phascolarctos, phascolomys, and perameles

Abstract

The occurrence of a cervical thymus gland in the Koala was first recorded by Symington (61) in 1900. This investigator examined one example of Phascolarctos cinereus measuring 30 cm. in length and found two thymus lobes in the neck lying in close contact with one another in the median plane, the right lobe slightly overlapping the left. The larger lobules were made up of cortical and medullary substance, in the last of which were Hassall's corpuscles. No thoracic thymus was present. Johnstone (26), in 1898, had previously examined two specimens of the same species measuring 20 cm. from the snout to the root of the tail, but in neither did he find any trace of a thymus either in the thorax or the neck. As regards the Wombat, Symington (60), in 1898, dissected three adult examples of Phascolomys, in all of which a cervical thymus was observed. It consisted of two lateral lobes placed just below the skin and the platysma, parallel with the median plane and overlapped by the submaxillary gland. On the right side of the thorax of one specimen an irregularly shaped thymic lobule, about 18 mm. long, was seen in some loose fat in front of the large vessels and the upper part of the pericardium, but no corresponding structure was present on the left. Symington at the same time remarked that the study of the development of the thymus ought to yield interesting results.