192 



E. H. NORRIS 



two parts are quite alike as regards regularity of outline and 

 smoothness of surface. The ventral surface of the tail is grooved 

 along nearly its whole extent by a closely applied blood-vessel. 



In embryos between 33 mm. and 50 mm. in length (which latter 

 approximately marks the end of the prefollicular period), the 

 gland continues its growth and increases considerably in size. 

 But notwithstanding the increasing mass of the organ, it con- 

 stantly maintains through succeeding stages an outline which 

 simulates that of the stage last described. 



One further observation on the external form should be noted. 

 Almost without exception, the glands, subsequent to the period in 



Fig. 2. Graphic reconstruction of the pharynx of an Acanthias em])ryo o mm. 

 long (No. 6). G.P.I, G.P.2, and G.P.3, first, second, and third gill pouches, 

 respectively; T., thyreoid. X 75. 



which they sever their connection with the pharynx, present a 

 small mound-like projection from the dorsal aspect of the body 

 (corpus). This little mound is quite regularly located in the 

 central region of this surface. 



b. Hisiogenesis 



The modifications in the external form of the gland are accom- 

 panied by, and to some extent at least, dependant upon a number 

 of internal changes. 



The anlage of the thyreoid gland when studied in cross-section 

 appears as a solid diverticulum or bud from the floor of the 



