igo 



THE SALAMANDER 



important Influence over the inception and course of metamorphosis 

 in Amphibia. 



2. The Thymus Glands. 



The thymus glands are paired from the first, and develop from the 

 dorsal ends of the third, fourth, and fifth gill slits (Maurer). Each 

 gland has therefore a triple origin and frequently presents a trilobed 

 structure in the adult. They are quite large ovoid glands, about 

 4 mm. long by 3 mm. broad, and are situated laterally on either side 

 of the 'neck' immediately beneath the skin, and about level with the 

 posterior margin of the paratoid cutaneous glands. They are of the 

 'solid' type of gland, and their cells, which are small, stain intensely 

 with iron-haematoxylin. They are supplied by the cutaneous branch 

 of the systemic arch (p. 201) and drained chiefly by the common 

 facial vein (p. 225). The function of the thymus in Amphibia does 

 not yet seem to be fully understood, but according to Noble (193 i), 

 'the thymus functions in producing lymphocytes, granulocytes, and 

 also erythrocytes to a certain extent'. It is apparently stimulated by 

 thyroid feeding. 



3. The Parathyroids (Epithelial Bodies) (Fig. 38, ep.). 



These bodies are also derivatives of the visceral clefts, notably the 

 third and fourth, but of their ventral ends. They are easily seen in 

 dissecting, lying lateral to the arterial arches immediately ventral to 

 the thymus, and consist of a pair of small spherical bodies on either 

 side. They are highly vascular and are usually supplied by small 

 twigs direct from the arterial arches. They appear late in larval life 

 and apparently have the same function in Amphibia as in Mammals, 

 namely to control the concentration of calcium salts in the blood 

 (Waggener, 1929). They are widely separated from, and have no 

 connexion with, the thyroids, although there is experimental evi- 

 dence to show that the removal of the thyroids in toads causes a 

 corresponding hypertrophy of the parathyroids. 



4. The Ultimo-branchial Body. 



This structure has also been variously termed post-hranchial hody^ 

 supra-pericardial body^ and ultimo-branchial body. The last name has 

 been adopted since it best indicates the origin of the structure from 

 the sixth gill-slit. In the adult Salamander it normally occurs on the 

 left side only, although in the Anura it is a bilateral structure. It is 

 quite small and is best observed in sections, where it may be seen 

 lying ventral to the M. cephalo-dorso-subpharyngeus and mesial to 



