THYREOID GLAND IN SQUALUS ACANTHIAS 



195 



this stage has the form of a pedunculated bud suspended from the 

 floor of the pharynx by a very narrow neck. Figure 4 is a cross- 

 section of the gland at this stage and shows the pedunculated 

 form as well as the structure of the bud. No cell boundaries are 

 apparent in this region, either in the entoderm lining the pharynx 

 or in the glandular epithelium. The nuclei are quite character- 

 istic in form, arrangement, and structure. As regards form and 

 arrangement, two general types of nuclei may be noted ; the first, 

 those which are elongately ovoidal and which are found in a cir- 



Fig. 4 Drawing of a cross-section through the thyreoid gland and adjacent 

 structures of an Acanthias embryo 5.8 mm. long (No. 9). Ec, ectoderm; H.C., 

 head cavities (outlined in stipple); L., lumen of pharynx; M., mitotic figure; 

 P., pigment granules. X 500. 



