224 THYROID BODY. 



arch. In front it bounded a groove (PI. XIV. fig. 5a, 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 (PI. 

 XIV. fig. 56, Tk). 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 meet- 

 ing 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 PL xviii. fior. 3, th., in 

 longitudinal and vertical section for a stage between 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 and the epithelium of the throat ; but the two are so close 

 together, that I am inclined to think that such a junction still 

 exists. It is certainly present up to stage P. 



