THYREOID GLAND IN SQUALUS ACANTHIAS 



203 



placed by follicles derived from them. Primary follicles con- 

 tinue to be formed until the embryo attains a length of approxi- 

 mately 250 mm. Apparently, by this time, the epithelial plates 

 have been completely used up in the formation of follicles. 

 At 250 mm. the thyreoid is made up of follicles which are 

 variable both in size and form. Apparently the follicles tend to 

 assume a spheroidal form, but through crowding and mutual 

 pressure of the adjacent elements, they are forced to assume 

 ovoidal or irregularly angular shapes (fig. 7). The follicle wall 

 consists of a single layer of low columnar epithelial cells which 

 show evidences of secretory activity. Colloid, which begins to 

 appear at about 160 mm. is abundantly present in the 250 mm. 

 stage. 



Fig. 7 Semidiagrammatic drawing of a cross-section through the thyreoid 

 gland of an Acanthias embryo 250 mm. long (No. 55), showing the relation of the 

 vascular sys|»em to the parenchymatous elements of the thyreoid. The colloid 

 which was present in nearly all of the follicles has not been shown in the drawing. 

 X 40. 



