TRITON 101 



right and left. The groups of vesicles of which it is composed 

 are surrounded by connective tissue. Close to it are the 

 parathyroids, on each side, and developed from the ventral 

 region of the gill-slits which close up at metamorphosis. Their 

 origin is therefore segmental, and it is worth noticing that 

 parathyroids do not appear in vertebrates with persisting water- 

 breathing gills. 



The thymus glands arise from the dorsal sides of the gill- 

 pouches, and are therefore also segmental in origin. 



The adrenal bodies are in a very interesting condition. 

 They consist of islands of tissue overlying the ventral surface 

 of the kidneys and extending forward as isolated lumps at the 

 side of the dorsal aorta. The bodies consist of two kinds of 

 tissue : cortical, corresponding to the inter-renal of the dogfish, 

 and medullary or chromaffme tissue, corresponding to the 

 supra- renals of the dogfish, and like them derived from the 

 sympathetic nervous system. The cortex (and inter- renal) is 

 formed from the ccelomic epithelium. In the region of the 

 kidney, the adrenal bodies are composed of both cortical and 

 medullary tissue, as in the higher vertebrates. The bodies in 

 front of the kidneys, however, may consist entirely of medullary 

 tissue, as in the supra-renals of the dogfish. These animals 

 therefore provide a very interesting intermediate condition. 



The pituitary consists of four parts. The pars nervosa 

 is formed from the floor of the infundibulum, the remaining 

 three (anterior, intermedia, and tuberalis) arise from the 

 hypophysis. 



Characters of Triton which show an advance over the con- 

 ditions in Fish, and which are typical of Tetrapoda : 



Limbs ending in digits (fingers and toes) ; 



Formation of arterial arches, short-circuiting the 



gill-capillaries ; 

 Interruption of dorsal aorta from internal carotid ; 

 Pelvic girdle composed of three elements : one dorsal 



and two ventral ; 

 Presence of an allantoic bladder ; 



