ORIGIN OF THE ENDOCRINE GLANDS 879 



The significance of this incorporation is unknown, and evidence of functional 

 thyroid tissue, being derived from the post-branchial body cells, is lacking. 



When the cellular masses of the developing thyroid gland reach the site of 

 the future thyroid gland, the cells multiply and break up into cellular strands, 

 surrounded by mesenchyme and blood vessels (fig. 366C). These strands in 

 turn break up into small, rounded, bud-like masses of epithelial cells, the young 

 thyroid follicles (fig. 366D). During the third month of development in the 

 human, colloidal substance begins to appear within the young thyroid follicles. 

 The colloid increases during the fourth month, and the surrounding cells of 

 the follicle appear as a single layer of low columnar cells. Each thyroid follicle 

 as a whole assumes the typical appearance of a functioning structure. Blood 

 capillaries ramify profusely between the respective follicles. 



The colloidal substance within each thyroid follicle presumably represents 

 stored thyroid secretion, and the thyroid gland is regarded, therefore, as a 

 "storage type" of endocrine gland. The theory relative to thyroid gland func- 

 tion is set forth that the follicle cells may secrete directly into the capillaries 

 and, hence, into the blood stream, or the secretion may be stored as colloid 

 within the follicles. Later this reserve secretion in the form of colloid may 

 be resorbed by the cells in times of extreme activity and passed on into the 

 region of the capillaries. In certain instances, e.g., dog and rat, individual 

 thyroid follicles may be lined with stratified squamous epithelium (Selye, '48, 

 p. 695). 



In the larvae of the cyclostome, Petromyzon, the so-called endostyle is lined 

 with rows of mucus-secreting cells, alternating with ciliated cells. This endo- 

 stylar organ becomes transformed into the thyroid gland upon metamorphosis. 

 A localization of iodine in certain of the endostylar cells in the larva has been 

 demonstrated (Lynn and Wachowski, '51, p. 146). 



4. Parathyroid Glands 



The parathyroid glands in man are four, small, rounded bodies, located 

 along the dorsal (posterior) median edges of the two thyroid lobes of the 

 thyroid gland (fig. 366B). Unlike the storage type of endocrine gland, such 

 as the thyroid gland with its follicles, the parathyroids contain no follicles 

 and, therefore, represent the solid type of endocrine gland. Blood capillaries 

 ramify through its substance which is composed of closely packed masses of 

 polyhedral epithelial cells, arranged in small cords or in irregular clumps. 

 Two main cell types are present in mammals, the chief or principal cells with 

 a clear cytoplasm and the oxyphil cells whose granules stain readily with acid 

 stains. The chief cells are common to all vertebrate parathyroids and thus 

 may represent the essential cellular type of the parathyroid gland (Selye, '48, 

 p. 540). 



The removal of the parathyroid glands results in a reduction of the calcium 

 content of the blood, muscular tetany, convulsions, and ultimate death. The 



