610 FRANCIS M. BALDWIN 



(fourth branchial) pouches respectively. The fourth pouch is 

 about 70 micra long, and the fifth, about 40 micra these two being 

 now comparatively close together. The anlage of the fourth 

 thymus body is fairly well defined. It is a lobate body, about 20 

 micra in diameter, extending dorsally from the caudal dorsal end 

 of the third pouch and, in one section, shows a tendency to be 

 constricted from the other cells of the pouch. The darker cells 

 are somewhat more compact about the dorsal tip of this anlage, 

 than in the case of the others. The vagus (X) ganglion is dorsal 

 and a little medial to the fourth anlage, while below and medial 

 to it are the pericardial chamber and the heart. It is difficult to 

 determine the exact extent of the anlage of the fifth thymus body. 

 It is a lobate, illy defined mass of cells extending in a dorsal 

 direction from the fourth branchial pouch towards the lateralis 

 ramus (L.) of the vagus nerve. 



The general positions of the thymus anlagen at this stage in 

 Amblystoma agree in most essentials with the description of the 

 bodies in Siredon and Triton by Maurer ('88), and in Siredon by 

 Driiner ('04) and Maximow ('12). Concerning the interpretation 

 of the darker cells which are found in close proximity, dorsal and 

 lateral, to the bodies there is some differences of opinion. Driiner 

 ('04), in his work on Siredon, was the first to suggest that they 

 are derived from the ectoderm, and that the}^ contribute to the for- 

 mation of the thymus bodies. To these groups he gave the name 

 'Ektodermzapfen.' Maximow ('12), p. 573, however finds them, 

 (der Driiner'schen Thymus ectodermalis) very sharply set apart 

 from the thymus bodies, and more closely associated with the 

 anlagen of the nerve ganglia of the different regions. He says, 

 "Ob diese Zellansammlungen wirklich dem Ektoderm entstam- 

 men, wie es Driiner will, vermag ich nicht anzugeben; jedenfalls 

 hangen sie alle unmittelbar oder durch einen kiirzeren oder lang- 

 eren Zellstrang mit der Masse der betreff enden Gehirnganglien zu- 

 sammen und sind von deren Zellen nicht scharf abzugrenzeii." 

 In my material, it is far from certain that there is any such ectoder- 

 mal contribution as Driiner claims. I find, indeed, cells, differing 

 in staining reaction from those of the other entodermal cells in 

 this very region, but I can trace no connection of these at any 



