880 THE DEVELOPING ENDOCRINE GLANDS 



parathyroid glands in some way regulate calcium metabolism to keep the 

 calcium content in the blood stream at its proper level. 



Parathyroid structures may be present in fish (Selye, '48), but it is gen- 

 erally believed that true parathyroid tissue is confined to the Tetrapoda. Two 

 parathyroid glands on each side are found in most urodeles and other am- 

 phibia, and in reptiles. The birds have relatively large parathyroid glands, 

 attached to the two thyroid glands located in the thoracic cavity. All mammals 

 possess parathyroid glands which, in some instances, are located internally 

 within the thyroid gland as well as externally. Accessory parathyroid glands, 

 apart from the two parathyroids attached to the thyroid gland, are found in 

 rats and mice and, consequently, may not be disturbed if the thyroid gland 

 is removed in these rodents. 



The parathyroid glands arise in the human embryo from proliferations of 

 the dorso-lateral walls of the third and fourth branchial pouches (fig. 366A). 

 The parathyroids which arise from the third pair of pouches are known as 

 parathyroids III, while those from the fourth pair of branchial pouches are 

 called parathyroids IV. Parathyroids III arise in close proximity to the thymus- 

 gland rudiments (fig. 366A). However, it is to be observed that the thymus 

 rudiments arise from the ventral aspect of the third pair of pouches. The 

 parathyroid-III rudiments move caudally with the thymus gland rudiments 

 and come to lie in relation to the lateral lobes of the thyroid, posterior to 

 parathyroids IV which take their origin in close relation to the post-branchial 

 (ultimobranchial) bodies (fig. 366A and B). 



Parathyroids IV appear to be a constant feature of all Tetrapoda. In those 

 species having but two parathyroids, it is probable that their origin is from 

 the fourth branchial pouches. 



5. Thymus Gland 



The thymus gland or "throat sweetbread" (the pancreas is referred to 

 commonly as the "stomach sweetbread") lies in the anterior portion of the 

 thoracic cavity and posterior neck region (fig. 366B). In some cases, it may 

 extend well along in the neck region toward the thyroid gland. In the thoracic 

 area, it lies between the two pleural sacs, that is, within the mediastinum, 

 and reaches as far caudally as the heart. Histologically, it is composed of 

 two parts: 



( 1 ) a cortex and 



(2) a medulla. 



The cortex contains masses of thymocytes or lymphocyte-like cells, while 

 the medulla contains thymocytes, reticular cells, and the so-called Hassall's 

 corpuscles, composed of stratified, squamous, epithelial cells. 



In man, the thymus gland arises from the ventral portion of the third 



