210 E. H. NORRIS 



6. DISCUSSION AND CONCLUSIONS 



The form of the thyreoid anlage in Squalus acanthias is of 

 interest. As pointed out in the section on organogenesis, the 

 anlage has the form of a localized, bud-Uke thickening of the 

 pharyngeal epithelium (figs. 1, 2, 17). There is no thyreoid 

 pouch or ' thyreoglossal duct' (His, '91) developed in any member 

 of the present series, as has been described by W. Miiller ('71) in 

 an Acanthias embryo and by other observers in other forms. This 

 may be a developmental peculiarity or, what seems more probable, 

 it may be that the relative growth changes in the cervical region 

 of the Acanthias embryo are less marked than they are in the 

 same region of various other vertebrates, so that instead of being 

 forced to change its relative position and to draw out a long con- 

 necting stalk, the gland remains in close relation to the pharyn- 

 geal floor until after its separation. The small mound-like 

 projection from the dorsal aspect of the body of the gland, found 

 in specimens immediately following the separation stage, prob- 

 ably marks the point of glandular detachment and may represent 

 the thyreoglossal duct of other forms. 



The fact that the relative position of the gland is changed very 

 little is clearly shown in figure 16 of the present paper and also 

 in a figure (fig. 3) published by Camp ('17). These figures indi- 

 cate that the principal topographical change is brought about 

 through the outgrowth of the long caudal portion of the gland, 

 and that, although there is a small amount of growth forward 

 from the anterior end of the body, there is almost no shifting of 

 the point of detachment from the level at which the gland was 

 originally set free. In other words, this point of detachment 

 seems to remain through successive stages at a point about mid- 

 way between the first and second gill clefts. 



Goodey ('10) studied the relations of the adult thyreoid gland 

 in three species of Selachians (Chlamdoselachus angineus, one 

 specimen; Scyllium catulus, three specimens; Scyllium canicula, 

 about thirty specimens) which are closely allied biologically to 

 Squalus acanthias. In some of these he found varying degrees 

 of connection between the thyreoid gland and the pharynx. In 



