Figure 12-2. Endocrine derivatives of the ptioryngeal pouches of different vertebrates. (After 

 Bertmor, 1961 ) 



The parathyroid masses become associated with the thy- 

 roid gland and generally are embedded in that gland. The 

 ultimobranchial body also becomes embedded in the thyroid. 



The thymus gland is of unknown function in the mammal. 

 Appearing to be lymphoid in nature, it is large in the young 

 and regresses in the adult. 



In the reptiles these lobular glands lie in the neck; in the 

 amphibians the thymus is small and lies behind the angle of 

 the jaw. In the sharks the thymus appears as a series of nod- 

 ules connected into a chain which lies above the gill 

 pockets, and it arises as thickenings of the branchial pouch 

 linings. There are six sets of nodules in Heplanchus, four in 

 most sharks. These thickenings may occur in the spiracular 

 and in the most posterior pouch of Spinax. In Heplanchus 

 the thymus nodules have the form of a bunch of grapes 

 with a duct opening into the pharyngeal pouch. 



In the actinopterygian fishes a rather massive thymus is 

 observed in the medial wall of the branchial cavity. This 

 thymus apparently arises from contributions of several of 

 the branchial pouches. The thymus of Latimeria is compa- 

 rable to that of the actinopterygian but is lobular. In the 

 cyclostomes all of the gill pouches give rise to thymus tissue 

 dorsally. The anterior pouches also have ventral anlagen. In 

 the adult only the dorsal parts remain. 



There are no parathyroid glands in the sharks or in the 

 actinopterygian fishes. Unquestionable parathyroids appear 

 first in the amphibians, and this may be correlated with the 

 loss of gills. It should be remarked here that a secretory or 

 endocrine function is assumed for the pseudobranch of 

 fishes, especially those buried in the tissue. In addition the 

 gills of marine teleosts have "chloride secreting" cells which 

 help in maintaining the internal osmotic state. Such se- 

 cretory cells could be the forerunners of endocrines. 



An ultimobranchial body is said to be present in the 

 sharks, holocephalans, and some ray-finned fishes. It some 

 of the teleosts, there is a flat discoidal mass lying in the con- 



nective tissue between the floor of the esophagus and the 

 sinus venosus. Experimental work with extracts of this tissue 

 suggests that it is parathyroid in function. It has also been 

 viewed as an accessory thyroid. The embryological source 

 of this tissue is in question but has been attributed to the 

 last gill pouches or the wall of the gut in the region of this 

 pair of pouches. In a few teleosts (characids), there are ac- 

 cessory branchial organs, diverticula of the fifth branchial 

 pouch, which appears to be endocrine in function. Paired or 

 unilateral ultimobranchial bodies lie in the connective tissue 

 dorsolateral to the anterior end of the pericardial cavity of 

 the salamander. The position of these bodies in the mam- 

 mal has been described. 



THE ADRENAL GLAND 



The adrenal gland of the mammal is compound, formed 

 from chromaffin cells of neural-crest origin and enclosed by 

 a capsule of splanchnopleure. The chromaffin cells form the 

 medulla. The secretion of this mass is similar to the adrenalin 

 (epinephrin) secreted by the sympathetic part of the auto- 

 nomic nervous system. The cortex of the gland is differen- 

 tiated into several layers and produces a number of steroid 

 secretions which are important in the functioning of the 

 body. 



In reptiles this may be a single organ on either side (croc- 

 odiles and turtles) or each may be separated into supra- 

 renal (chromaffin) and interrenal (epithelial) components. 

 In birds and amphibians a single pair of adrenal glands is 

 present. In the lungfish Protopterus, interrenal and chromaf- 

 fin tissues are intermingled and are located along the venous 

 channels of the ventral side of the kidney. 



In actinopterygian fishes there is a range from a fairly 

 compact adrenal gland in Cottus. where chromaffin cells are 

 massed within the interrenal tissue, through Pleuronectes. in 

 which the two components are more or less closely associated, 



THE ADRENAL GLAND 



381 



