PHARYNGEAL DERIVATIVES OF AMBLYSTOMA 651 



Thyreoid. Although the general anatomy and position of the 

 thyreoid gland in certain adult amphibians was noted by a few 

 workers in the latter half of the last century, notably — Leydig 

 ('53) in Triton punctatus, 'Landsalamander/ Proteus, Caecilia 

 annulata, Rana and Bufo, and Wiedersheim ('79) in Caecilia, 

 Siphonops and Epicrium, and later ('84) the same author noted 

 its position in representatives of both anura and urodelia — the 

 first investigator to describe it from the developmental side was 

 W. Miiller ('71) who followed its ontogeny in Rana temporaria 

 and R. platyrrhinus. Soon after this, Goette ('75) described its 

 development in Bombinator igneus, and a little later, de Meuron 

 ('86), worked it out in both Rana and Bufo. According to 

 Maurer ('88), no work had been done on the development of the 

 thyreoid gland in the urodeles before he wrote, and what was 

 known of it was learned from macroscopic study of the adult 

 animal. As a basis of his study, he used series of Triton taenia- 

 tus, T. alpestris and T. cristatus (stages in metamorphosis ex- 

 cepted), supplemented by series of Salamandra maculata and 

 young and old axolotls. He also studied representatives of the 

 anurans — Rana esculenta and temporaria; Bufo vulgaris and 

 variabilis; Hyla viridis and Bombinator igneus. His results are 

 briefly as follows : 



In Triton taeniatus the thyreoid anlage appears before the gill- 

 pouches have united with the ectoderm, as a solid epithelial proc- 

 ess, extending ventrally from the floor of the pharynx, between 

 the second gill-pouch and the pericardial cavity. During the first 

 day "after hatching (5.5 mm. long), the anlage separates from its 

 point of origin, as an ovoid mass of yolk-filled cells. By the 

 growth of the copula and the other structures in its region, the 

 anlage gradually splits longitudinally into right and left halves 

 except for a small portion in front which persists for some time 

 as the 'isthmus,' holding the two halves together. Eight days 

 later, (larva now 7 mm. long), division is complete, the two halves 

 being now separated by the developing sternohyoid muscle. 

 From this point on irregularities occur, both in size and in details 

 of the two halves, and the color of yolk content of the cells. A 

 few cells (from four tp six) may group themselves in an irregular 



