216 E. H. NORRIS 



As to the further steps in the process of morphogenesis of the 

 follicles from these anastomosing 'cords/ widely divergent views 

 have been held. Inasmuch as the process of follicle genesis has 

 been found to be practically identical in Squalus acanthias with 

 the process described by the author in the human thyreoid 

 (Norris, '16), the reader is referred to this earlier work for the 

 discussion of the changes noted and for a consideration of the 

 various views presented by the literature as regards this matter. 



The relative magnitude of the vascular bed which is associated 

 with the gland, the close relationship which is established between 

 the blood stream and the parenchymatous elements, and the com- 

 paratively small amount of arterial blood furnished to the gland 

 are the most noteworthy conditions in connection with the blood 

 supply of the thyreoid. 



As shown in figure 14, the gland is literally suspended in a lake 

 of blood, the blood being separated from the epithelial structure 

 only by a single layer of epithelial cells. 



7. SUMMARY 



1. The thyreoid gland in Squalus acanthias makes its appear- 

 ance in embryos of approximately 4 mm. in length, as a solid 

 epithehal bud from the floor of the pharynx. 



2. The bud grows caudally and becomes keel-shaped. At this 

 stage it is j oined to the pharynx by a very short and narrow stalk. 



3. By the time the embryo has attained a length of 19 mm. 

 the gland has severed its connection with the pharynx and has 

 the form of a column with rounded ends. 



4. In no case was a ' thyreoglossal duct' observed. 



5. The gland soon loses its columnar form, and as it gradually 

 flattens out dorsoventrally, becomes lozenge-shaped in outline. 



6. In embryos of about 30 mm. the gland for the first time 

 assumes in miniature the form of the adult gland. It may be 

 divided for descriptive purposes into two portions: an anterior 

 part (corpus), which is rhomboidal in outline and relatively thick 

 in cross-section, and a posterior part (cauda) , which is relatively 

 thin in cross-section and has the form of a long straight bar 



