§ 2.222 ENDODERMAL ENDOCRINE GLANDS 49 



into it as droplets from the cells (Figs. 2- 14a and 4-7). This appears 

 to be a precursor of the hormone, usually in the form of diiodo- 

 tyrosine ; it is later reabsorbed into the cells, converted to thyroxine, 

 and passed into the blood as the hormone (Fig. 4-8). This process 

 is described more fully in relation to its control by the thyrotrophic 

 hormone, TSH (§ 4.221). 



Ultimohranchial bodies 



These structures arise ventrolaterally from the epithelium of the 

 last gill slit and may be seen in development to give rise to a pair 

 of small glands. In fish they appear to replace the parathyroid 

 glands ; but in tetrapods they may be present in addition to them 

 (Fig. 2-13). In function they appear to be similar to the para- 

 thyroids (§ 5.4). 



Parathyroid glands 



These glands are serially homologous w^ith the ultimohranchial 

 bodies behind (and with the so-called carotid gland of Amphibia 

 in front). They arise ventrolaterally from the epithelium lining 

 the gill slits on the anterior surfaces of the fourth and fifth visceral 

 arches (Fig. 2-13). 



The individual cells of the parathyroid glands form a densely 

 packed mass amid ramifying blood vessels, and are not arranged in 

 follicles. They may be of two kinds: the more numerous have 

 clear cytoplasm and relatively large nuclei ; the rest are acidophil 

 with granular cytoplasm. The former are thought to be the main 

 source of parathormone (§ 5.4). 



2.222 Gland cells in the pancreas 



In most vertebrates, groups of endocrine cells occur among the 

 exocrine cells (secreting enzymes and alkali) in the pancreas ; they 

 form the islets of langerhans. In larval lampreys these endo- 

 crine cells lie adjacent to the rest of the pancreatic tissue, and only 

 become embedded within it in the adult. The endocrine cells are 

 distinguishable histologically because they stain much less readily 

 than the surrounding exocrine gland cells. This is due to the 

 constant presence of so-called beta (^) cells that secrete the anti- 

 diabetogenic hormone, insulin (§ 5.212). These cells have very 



