THE THYROID BODY. 453 



embryo it appeared as a diverticulum from the ventral surface 

 of the throat in the region of the mandibnlar arch, and extended 

 from the border of the mouth to the point where the ventral 

 aorta divided into the two aortic branches of the mandibular 

 arch. In front it bounded a groove (PI. 15, fig. $a, Th.}, directly 

 continuous with the narrow posterior pointed end of the mouth 

 and open to the throat, while behind it became a solid rod 

 attached to the ventral wall of the oesophagus (PL 15, fig. $b, 

 Th.}. In a Scyllium embryo belonging to the early part of 

 stage K, the thyroid gland presented the same arrangement as 

 in the Torpedo embryo just described, with the exception that 

 no solid posterior section of it was present. 



Towards the close of stage K the thyroid body begins to 

 elongate and become solid, though it still retains its attachment 

 to the wall of the oesophagus. The solidification is effected by 

 the columnar cells which line the groove elongating and meeting 

 in the centre. As soon as the lumen is by these means obliterated, 

 small cells make their appearance in the interior of the body, 

 probably budded off from the original columnar cells. 



The gland continues to grow in length, and by stage L 

 assumes a long sack-like form with a layer of columnar cells 

 bounding it externally, and a core of rounded cells filling up its 

 interior. Anteriorly it is still attached to the throat, and its 

 posterior extremity lies immediately below the end of the ven- 

 tral aorta. The cells of the gland contain numerous yellowish 

 concretionary pigment bodies, which are also present in the later 

 stages. 



Up to stage P the thyroid gland retains its original position. 

 Its form and situation are shewn in PI. 19, fig. 3, th., in longitu- 

 dinal and vertical section for a stage between O and P. The 

 external layer of columnar cells has now vanished, and the gland 

 is divided up by the ingrowth of connective-tissue septa into a 

 number of areas or lobules the rudiments of the future follicles. 

 These lobules are perfectly solid without any trace of a lumen. 

 A capillary network following the septa is present. 



By stage Q the rudimentary follicles are more distinctly 

 marked, but still without a lumen, and a connective-tissue sheath 

 indistinctly separated from the surrounding tissue has been 

 formed. My sections do not shew a junction between the gland 



