THYREOID GLAND IN SQUALUS ACANTHIAS 213 



the thyreoid of Seyllium embryos, and figured by Baumgartner 

 ('15) in the hypophysis of Squalus acanthias. The pigment does 

 not seem to be associated with nuclear or cellular degeneration, 

 although in some cases the pigment blocks are superimposed on, 

 and nearly obscure certain nuclei. The fact that this material 

 is found in the ectoderm in the region near which the oral plate 

 has opened, and in the thyreoid, chiefly in the neck region of the 

 gland, suggests that it may be associated with any division or 

 separation of epithelia. 



After the gland has established its independence from the 

 pharynx, its structure is again' altered, as described above, by 

 the development of completely closed cavities within the thicker 

 parts of the gland mass. These cavities, although not previously 

 described in the Selachian thyreoid, apparently correspond to the 

 lumen which W. Miiller (71) mentions as present in the thyreoid 

 of one of his Acanthias embryos, and to the cavities which 

 Scammon ('11) observed in three specimens in the Normal-plate 

 series (No. 29, 31, 32). 



Since the gland has been regularly found to be sohd up to the 

 time of its separation from the pharynx, and since no thyreoid 

 pouch or thyreoglossal duct has been observed, it is evident that 

 these cavities develop quite independently of any earlier pouch, 

 duct, or cavity. Further evidence in favor of this conclusion may 

 be drawn from the fact that a number of these closed cavities 

 or spaces of various sizes have been found in different stages of 

 development not only in many of the glands, but also in different 

 parts of the gland in the same specimen. 



The cavities, which I first observed in the human thyreoid 

 (Norris, '18) have been subsequently found in the thyreoid of 

 Acanthias embryos (Norris, '17). In the human embryos, the 

 cavities appear at a stage in the glandular development which 

 exactly corresponds to the stage of their genesis in the fish 

 thyreoid. Moreover, the role they play in the two forms is 

 apparently identical. Born ('83), in describing the thyreoid 

 glands of pig embryos 7 mm. long, mentions the presence of lu- 

 mina in the lateral end of the gland mass. He does not include 

 any description of these lumina, but they probably correspond to 



