§ 2.221 ENDODERMAL ENDOCRINE GLANDS 47 



two groups (§ 2.221), and the pancreas, where they form the islets 

 of Langerhans (§ 2.222). These glands all secrete metabolic 

 hormones, and their structure has long been known; it is well 

 described in most text-books of vertebrate anatomy, embryology 

 and histology, and little need be said here, except to emphasize the 

 fact that they can be identified in most classes of vertebrates and 

 are not confined to the warm-blooded forms, like the sources of 

 the gastrointestinal hormones. 



2.221 Glands of the pharynx 



These are the thyroid and parathyroid glands and the 

 ULTIMOBRANCHIAL BODIES, which are the homologues of the para- 

 thyroids (Table 5). The thymus glands also arise here (Fig. 2-13); 

 but their endocrine nature is uncertain. It will be considered in 

 relation to growth (Part II, § 3). 



Thyroid glands 



In Agnatha, the endostyle in the floor of the larval pharynx can 

 accumulate iodine, and even synthesize thyroxine, albeit in small 

 quantities, even before its transformation to the adult thyroid 

 gland. The endostyle of the amphioxus, Branchiostomay also 

 accumulates iodine ; but it does not appear to synthesize thyroxine 

 (Barrington, 1958). It is then possible to argue either that the 

 endostyles of the Protochordates and the Agnatha are homologous 

 with each other and with the thyroid gland, because the former 

 are structurally similar although only the two latter have acquired 

 the ability to synthesize thyroxine, or that a true homology 

 should be marked by similar chemical activities, and is in this case 

 limited to the vertebrates. 



The thyroid gland retains its position as a single median organ 

 in the floor of the pharynx in lower vertebrates (Fig. 2-13) where 

 the gland itself may be diffuse, as in many teleost fish, or relatively 

 compact and enclosed in a capsule of connective tissue, as in 

 Elasmobranchii and a very few Teleostei, such as Pseudoscarus, 

 from which it can therefore be relatively easily removed. In 

 tetrapods, in which the gills are lost, the gland often becomes 

 paired and may even be further subdivided; but each part is 



