THYREOID GLAND IN SQUALUS ACANTHIAS 217 



joined with the posterior angle of the corpus in front and termi- 

 nated caudally in a pointed extremity. 



7. Except for the indefinite vacuole-Hke spaces which are 

 found in the thyreoid of embryos ranging from 5 to 20 mm. in 

 length, the gland is quite solid throughout the period of its early 

 morphogenesis. 



8. At about the stage (20 mm.) when the vacuole-like spaces 

 cease to be found, there are formed within the thicker portions of 

 the gland a number of completely closed cavities, which in con- 

 trast to the earlier vacuole-like spaces are outlined by very definite 

 margins. These intraglandular^cavities are very tiny when they 

 first appear, but as they increase in size they may open into one 

 another, thus transforming the gland from a solid into a hollow 

 organ. 



9. At a later stage (28 mm.) these cavities for the first time 

 open to the outside and are invaded by blood-vessels. 



10. The process of cavity formation is accompanied or perhaps 

 preceded by four other processes, as follows: 1) differentiation 

 of cell boundaries; 2) changing form of the nuclei (cells); 3) 

 changed position of the nuclei (cells) ; 4) cell proliferation. 



11. The intraglandular cavities are regularly surrounded by 

 epithelial plates which are two cells in thickness, so that when 

 the cavities are opened to the outside the gland is transformed 

 into a number of smooth epithelial plates which anastomose 

 with each other freely and which are regularly two cells thick. 



12. The primary thyreoid follicles arise directly as isolated 

 and independent structures from the epithelial plates of the pre- 

 follicular period, by the rearrangement of cells, cell proliferation, 

 increase in the size of the cells, and lumen formation. 



13. The primary follicles appear in embryos of about 60 mm. 

 in length and continue to be formed until the embryo attains a 

 length of approximately 250 mm. 



14. There are no secondary follicles found before 250 mm. 



15. Branches of the external jugular vein invade the region of 

 the thyreoid and begin to form a vascular plexus around the gland 

 in embryos about 19 mm. in length. 



