312 



HISTOLOGY 



sec.-' 



in the presence of one or the other of these different kinds of 



granules. The cells pass the 

 secretion into the lumen of 

 the lobule, which has no duct 

 or means of discharging the 

 mass externally. It is retained 

 as a colloid substance in the 

 lumen of the lobule. There is 

 a constant growth of the 

 younger lobules, which appear 

 at certain germinal centers 

 and slowly increase in size. 

 Figure 280 shows several of 

 the cells which line one of the 

 larger lobules in this fish. 



The thyroid gland is usually 

 accompanied by another struc- 

 ture, the parathyroid gland or 

 body. In most of the higher 

 vertebrates the parathyroid ap- 

 pears as several small irregu- 

 lar bodies, the lobes, which lie 

 on the posterior edge of the 



thyroid. They vary in form and 'position, and may be two or four in 



number according 



as one or both of 



the last two pairs of 



gill clefts took part 



in their formation. 



Each lobe of this 



tissue is invested 



with a dense connec- 

 tive-tissue sheath, 



and its interior retic- 



ulum of loose con- 

 nective tissue is filled 



with two kinds of 



cells which appear 



to be specific to the 



gland. The most 



abundant of these 



are called the prin- 

 cipal cells. They are slightly irregular ovals or spheres, with a large 



FIG. 280. Vertical section of a small part of the 

 epithelium which lines one of the lobules of the 

 thyroid gland of a skate, sec., secretion in lumen ; 

 bl.v., blood vessel in connective tissue that sur- 

 rounds the lobule. X 1 100. 



FIG. 281. Bit of tissue from the parathyroid gland of a young 

 cat. Two blood vessels are seen, near one of which is a single 

 acidophile cell (ac.c.). X 750. 



