THYREOID GLAND IN SQUALUS ACANTHIAS 



199 



follicle will be used to include those follicles developing inde- 

 pendently of pre-existing follicles. The follicles derived from 

 pre-existing follicles, by budding or otherwise, are termed second- 

 ary follicles. 



In the present series, the first primary thyreoid folhcles appear 

 in an embryo 60 mm. long (No. 47). In this specimen the 

 thyreoid gland has essentially the same structure as has that of 

 the last stage described in the prefoUicular period, i.e., it is made 

 up chiefly of longitudinally placed epithelial plates or bands, 

 which are only two cells in thickness. But in this stage, the 

 plates, which have in previous stages been characterized by com- 



L.C. 



Fig. 6 Small portion of a cross-section of the thyreoid gland in an Acanthias 

 embryo 160 mm. long (No. 50), magnified to show cell structure and the relation 

 of the developing primary follicles to the epithelial plates. Note the appearance 

 of lumina (L) in buds {B) from the surface of the plate as well as in swellings 

 along its course (L'). B.C., blood corpuscle; E.G., endothelial cell. X 450. 



paratively smooth surfaces, now present surfaces which are more 

 or less roughened by the appearance of scattered hillocks or 

 mounds (figs. 6, 23). They are placed very irregularly with 

 respect to one another, and may appear for the first time in any 

 part of a plate, at its periphery or in a more central region. 



Cross-sections (fig. 6) show that these hillocks are the imme- 

 diate anlagen of the early thyreoid follicles. Further, the hillocks 

 are apparently produced by the concurrence of four different 

 processes in the epithelium. The first process is that of cell 

 rearrangement, the second that of cell proliferation, the third that 



