Development of Thymus, etc., in Marsupialia. 101 



except for one foetus, where a thymus III is developed. The second 

 epithelial body, when present, owing to its posterior position and to its- 

 occasional occurrence close to the ultimobranchial body and the lateral lobe 

 of the thyroid, may be regarded as having originated from the fourth pouch 

 and therefore corresponds to epithelial body IV. 



The thyroid, as in Trichosurus, develops chiefly from a median primordium. 

 The ultimobranchial body here also becomes closely connected with the 

 lateral lobes and gives off sprout-like processes which probably contribute to 

 the formation of thyroid tissue. In Phascolarctos, the lateral lobes are remark- 

 able in their tendency to be asymmetrical and to become divided up into 

 several isolated portions. 



In the possession of only one pair of superficial cervical thymus glands, 

 Phascolarctos and Phascolomys exhibit a noteworthy parallelism with the 

 mole, in which also the cervical thymus alone remains as the functional gland 

 of the adult. 



Perameles. 



In Perameles, there is no superficial cervical thymus at any stage of 

 development, but two pairs of well-developed thoracic glands are always 

 present, as are also two epithelial bodies, viz., epithelial bodies III and IV. 



The thymus is derived certainly from the ventral solid portion of pouches 

 III and IV, the epithelial body arising from the dorsal portion of the pouch 

 in each case, but whether the primordium of thymus III also takes origin, as 

 in Trichosurus, from the caudal wall of the dorsal part of the pouch, could not 

 be determined in the material available. The thymus glands attain a. 

 remarkably large size and may unite to form a single thymus on each side or 

 may remain quite separated from each other. 



The median thyroid primordium and the ultimobranchial body differentiate 

 as in Trichosurus, but the evidence in favour of the participation of the 

 ultimobranchial body in the formation of thyroid tissue is especially 

 convincing in Perameles. 



