Development of Parathyroids 21 



itself take on a similar appearance, becoming large and irregular (fig. 16). 

 In this case there is also seen a multiplication of intervesicular cells, which 

 the authors believe to be of the same nature as the cells of the parathyroids. 

 The conclusion they arrive at is that the tissues of the two organs are 

 largely intermixed, at least in birds and mammals. In lower Vertebrates, 

 however, Mrs Thompson found the two to be completely separate both 

 developmentally and structurally. 



The parathyroids are amongst the most vascular organs in the body. 

 They are supplied each by a special branch of the inferior thyroid artery. 



Fig. 16. — Section of thyroid of dog, thirty-two days after removal of all the para- 

 thyroids. (Halpenny and Thompson.) The appearance is singularly like that of 

 the parathyroid shown in fig. 15, and somewhat resembles the thyroid of ex- 

 ophthalmic goitre (fig. 26). 



»«*., vesicle ; e.ves., epithelium lining vesicle; e.interves., intervesicular tissue ; c.t., connective tissue. 



The sinus-like capillaries come into close relationship with the epithelial 

 cells of the gland. 



The nerves of the parathyroids, like those of the thyroids, pass both 

 to the vessels and to the secreting cells. Some evidence has been adduced 

 which seems to show that the cell-activity is controlled by the nervous 

 system. 



Development. — The parathyroids are developed as outgrowths of the 

 Illrd and IVth visceral pouches on each side (fig. 2), that from the 

 Illrd pouch giving rise to the upper, and that from the IVth to the lower 

 parathyroid. From the same pouches the thymus is also derived; occasionally, 

 as already mentioned, one or more outgrowths from the thymus rudiments 

 pass into the thyroid. On the other hand, parathyroid and even thyroid 

 tissue occasionally occurs within the thymus ; in some animals constantly. 



