212 E. H. NORRIS 



one case he found a tube-like structure extending from the 

 pharynx through a foramen in the basihyal cartilage to the 

 anterior end of the thyreoid, and in three other cases he found 

 typical thyreoid tissue within the foramen of the basihyal carti- 

 lage. He concludes that this foramen "must have been occupied 

 by the original entodermic evagination." These results of 

 Goodey receive no support from the present investigation, for the 

 thyreoid gland in Squalus acanthias has completely severed its 

 connection with the pharynx some time before the basihyal 

 cartilage is laid down. However, certain possibilities suggest 

 themselves in attempting to harmonize these apparently con- 

 flicting observations. It may be that the thyreoid of the forms 

 described by Goodey actually undergoes a somewhat different 

 developmental history from that described in the present paper, 

 or it may be that the cases in which he found adventitious thyre- 

 oid tissue are exceptional and represent cases of delayed separa- 

 tion of the gland. 



The transformation of the gland from its columnar form (figs, 

 4, 19) to a plate-like structure, flattened dorsoventrally (figs. 5, 

 10, 11, 20, 21, 22, 23), is apparently due to pressure exerted by the 

 development of surrounding elements, the chief of which are 

 the basihyal cartilage above and the ventral musculature below. 

 What may be the significance of the vacuole-like spaces which 

 appear within the early gland (fig. 3) cannot be said. Also an 

 explanation of the meaning of the difference in form of the nuclei 

 of the central and peripheral portions of the gland in these early 

 stages (figs, 3, 4) is wanting, Balfour ('85), in describing the 

 structure of the thyreoid of a Scyllium embryo which belongs in 

 his stage L, points out a similar arrangement of the cells, but 

 offers no suggestion of its meaning. 



The large amount of granular pigment found in the parenchyma 

 of the gland is a striking feature (fig. 4). This pigment increases 

 in amount up to the time of separation of the gland from the 

 pharynx, and thereafter rapidly disappears. It is massed in 

 largest quantity at the point at which separation is to take 

 place. Similar pigment is to be seen in the ectoderm ventral to 

 the thyreoid region, and was also described by Balfour ('85) in 



