THYREOID GLAND IN SQUALUS ACANTHIAS 191 



ed are nearly coextensive. By the time the embryo has reached 

 14 mm. in length the thyreoid is better described as keel-shaped 

 (fig. 18). The gland mass and the suvspending neck are no longer 

 coextensive, for the former has grown caudally until it is nearly 

 three times as long as the latter. 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 

 and whose cross-section is ovoid (fig. 19). But the gland does 

 not long preserve its columnar form. By the time the embryo 

 has reached a length of 24 mm. the gland is well advanced in its 

 transition, being neither column/ar, as was the condition at 19 

 mm., nor completely flattened, as in the succeeding stages. This 

 transitional stage is well set forth in figure 20. At 28 mm. the 



GP2 



G.Ri. 



Fig. 1 Graphic reconstruction of the pharynx of an Acanthias embryo 3.8 

 mm. long (No. 3). G.P.I, and G.P.2, first and second gill pouches, respectively; 

 r., thyreoid. X 75. 



gland has cjuite lost its columnar form of the earlier periods and 

 has assumed the form of a flat, diamond-shaped plate whose 

 surfaces are quite smooth and regular (fig. 21). 



Between 28 and 33 mm. the gland again changes in form, and 

 only at this stage (33 mm.) can it be said of the organ for the first 

 time that has in miniature the form of the adult thyreoid (fig. 

 22). For descriptive purposes the gland is readily divisible 

 into two parts, an anterior, the corpus, and a posterior, the cauda. 

 The corpus, as seen from above or below^ is rhomboidal in outline, 

 its transverse diagonal being a little greater than its anteropos- 

 terior ; while the cauda, on the other hand, presents the form of a 

 long straight bar which joins directly with the posterior angle of 

 the corpus in front and terminates posteriorly in a pointed extrem- 

 ity. The body is nearly three times as thick as the tail, but the 



