Fig. 8 Cross-section through the pharyngeal region of an Acanthias embryo 

 5.2 mm. long (No. 7) . The thyreoid bud is shown attached to the ventral wall of the 

 pharyn.x. The first aortic arch of either side lie in contact with the lateral sur- 

 faces of the bud. The anterior head cavities lie lateral to the aortic arches. 

 The pharynx, the second gill clefts, the dorsal aortae, and the lower half of the 

 notochord are also indicated. 



Fig. 9 Cross-section through the mandibular process of an Acanthias embryo 

 14 mm. long (No. 24). The thyreoid gland is shown as a pedunculated bud 

 attached to the pharyngeal floor. The aortic arches are separated from the thy- 

 reoid bud by a considerable mass of mesenchyma. Two anterior head cavities 

 are located ventrolaterally to the thyreoid. 



Fig. 10 Cross- section through the ventral w^all of the pharynx of an Acanthias 

 embryo 28 mm. long (No. 38). Thyreoid gland is shown near the center of the 

 figure as a flattened structure having two closed cavities within. A considerable 

 amount of mesenchyma is present in which blood-vessels, muscles, and cartilage 

 are being developed. The vessels are a part of the thyreoid plexus of venules. 

 The most ventral circular muscle is the constrictor pharyngis. The longitudinal 

 bundle just ventral to the thyreoid is the coraco-mandibularis. The dense 

 mesenchyma dorsal to the thyreoid is the anlage of the basihyal cartilage. The 

 most dorsal and ventral structures are entoderm and ectoderm, respectively. 



Fig. 11 Cross-section of the ventral wall of the pharynx of an Acanthias 

 embryo 33 mm. long (No. 40). Thyreoid gland is shown near the center of the 

 figure as a flattened structure, having four cavities, all of which are invaded by 

 blood-vessels. Blood-vessels have increased in size and number around the gland 

 and have associated themselves more closely with its surfaces. The surfaces of 

 the epithelial plates of the thyreoid are quite smooth. The ectoderm, constrictor 

 pharyngis, and coracomandibularis muscles, the basihyal cartilage, and the 

 entoderm have positions similar to those of the same structures in figure 10. 



Fig. 12 Cross-section of a part of the ventral wall of the pharynx of an 

 Acanthias embryo 48 mm. long (No. 45). Thyreoid gland is shown near the center 

 of the figure as a structure made up of anastomosing epithelial plates which are 

 quite regular in thickness and which have smooth surfaces. Blood-vessels have 

 increased in size and number around the gland and have intimately associated 

 themselves with its surfaces. The other structures shown are located similarly 

 to the same structures in figure 11, with one exception. The left coracohyoid 

 muscle is located between the thyreoid gland below and the basihyal cartilage 

 above. 



Fig. 13 Cross-section of a part of the ventral wall of the pharynx of an Acan- 

 thias embryo 95 mm. long (No. 49.). Thyreoid gland is shown near the center of the 

 figure as a structure which is made up of epithelial plates which have rough sur- 

 faces. Follicles are being formed in the plates and a few follicular lumina may 

 be seen. The blood-vessels have fused around the gland to form the thyreoid 

 sinus within which the gland is suspended. Note the two valve leaflets at the 

 left end of the sinus. The other structures shown are located similarly to the 

 same structures in figure 11. 



Fig. 14 Cross-section of a part of the ventral wall of the pharynx of a new- 

 born specimen of Squalus acanthias 170 mm. long (No. 52). The thyreoid gland^ 

 is shown near the center of the figure as a structure which is made up of follicles 

 and a few plates in which follicles are being formed. The magnitude of the 

 thyreoid sinus and the fact that the thyreaid is suspended within it from only 

 two points are striking features. Note the valve leaflet at the left end of the 

 sinus. The other structures shown are located similarly to the same structures in 

 figure 11. 



208 



