204 E. H. NORRIS 



Up to and including the 250 mm. stage, no evidences of the 

 formation of secondary folUcles have been observed. The 

 material available from stages later than those included in the 

 table of embryos studied was not suitable for histologic study. 

 However, the thyreoids from three adult specimens were ob- 

 tained and, although the glands were poorly preserved, certain 

 observations were possible. A number of irregular follicles with 

 bud-like processes are present which suggest the formation of 

 secondary follicles. 



5. BLOOD SUPPLY 



From the time of its first appearance the thyreoid gland in 

 Squalus acanthias is associated more or less intimately with the 

 vascular system. In the early embryos, in which the gland is 

 just recognizable, the ventral aorta bifurcates immediately caudal 

 to the gland, so that its two most anterior branches (first arches) 

 pass dorsally on either side of the thyreoid. These thin-walled 

 vessels lie in immediate apposition with the thyreoid anlage 

 (fig. 8), and although they give off no branches to the gland, they 

 probably suffice to nourish all the tissues of this region. 



Soon after these vessels have associated themselves with the 

 thyreoid, mesenchyma begins to invade the region and to grow 

 around both the vessels and the gland, so that by the time the 

 embryo has reached a length of 5.8 mm. (No. 645) the mesen- 

 chyma, through its rapid growth, has formed a dense tissue which 

 separates the blood-vessels from the gland for a considerable 

 distance (fig. 9) . This relation between the gland and the vascu- 

 lar system is maintained until the time at which the gland is cut 

 off from the pharynx (ca. 19 mm.). At this stage another set 

 of vessels begin to invade the region in which the thyreoid is 

 placed. These are thin-walled venules growing forward from a 

 trunk which buds from the common cardinal vein just before it 

 opens into the sinus vJfcosus. This venous trunk (external 

 jugular or linguofacial vein) buds from the common cardinal in 

 embryos of about 14 mm. in length. It grows downward along 

 the side of the pericardium and comes to lie ventral to the heart 

 and to the thyreoid. It breaks up into a wide-meshed network 



