190 



E. H. NORRIS 



1 In correlating emljryos of the present series with Normal-plate numbers and 

 with Balfour's stages only the general development of the embryo has been con- 

 sidered and not the state of development of the thyreoid gland. 



3. PREFOLLICULAR PERIOD 



a. Organogenesis 



The anlage of the thyreoid gland in Squalus acanthias makes its 

 appearance in embryos of approximately 4 mm. in length as a solid 

 epithelial bud from the floor of the pharynx. This bud, which is 

 at first little more than a localized thickening of the entodermal 

 lining of the pharynx, is placed just ventral to the point at which 

 the oesophagus leads from the pharynx and in the region inferior 

 and caudal to the ventral extremities of the first two pairs of gill 

 pouches (figs. 1, 2, 17). A study of these figures will show the 

 absence of any groove or pouch in the floor of the pharynx at the 

 point where the thyreoid first appears. Not only is there an entire 

 absence of a groove or pouch, but, as shown in figures 1 and 2, 

 the anlage even protrudes somewhat into the cavity of the 

 pharynx. The bud increases in size and, extending caudally, 

 assumes a pedunculated form, being suspended from the floor 

 of the pharynx by a rather extensive but very narrow neck. 

 This condition obtains in embryos of 5.8 mm. (No. 9 of present 

 series), where the gland mass and the neck by which it is suspend- 



