238 Henry Fox 



is more nearh' vertical than in the preceding stages. Its distal extremity 

 is somewhat bulb-like in form (Fig. 39, la. t.), but its basal portion has 

 become constricted to a narrow duct, opening into the sinus piriformis 

 (Fig. 40). The dorsal portion is much reduced; it is represented only 

 by the dorsal process, which forms a spheroidal body attached by a 

 narrow stalk to the duct (Figs. 14, 16, Gl.T.). It contains only a slight 

 lumen. Otherwise it forms a solid mass, whose walls are apparently 

 thrown into series of tight folds. These produce an appearance simu- 

 lating that of the carotid gland — an organ produced from the homol- 

 ogous part of the preceding j)ouch. 



At this period the distal extremity of the ventral diverticulum is 

 without any connection with the median tliyroid. The latter at this 

 time is rather small and lies in the mesial plane above and between the 

 ventral extremities of the thymus downgrowths. 



In both the 17 and 18 mm. pigs the ventral diverticulum has still 

 further elongated, largely as a result of the lengihening of its duct, 

 which now appears as a slender, solid cord (Figs. 19, 21). A lumen is 

 present only in the terminal vesicular part. The dorsal process (Gl.T.) 

 presents the same appearance as before; its lumen, however, has dis- 

 appeared and its structure more clearly resembles that of the carotid 

 gland (Fig. 45, c.gl.). We shall henceforth designate it the "glandule 

 thyroidienne" — a term applied to it by Prenant. 



The median thyroid now occupies a position considerably posterior to 

 that occupied by it ^in the preceding ststge. It has grown considerably 

 and has assumed a horseshoe shape, owing to the outgrowth of its lateral 

 wings, which at their outer extremities almost touch the vesicles of the 

 ventral diverticulum. 



In a cat of 10.7 mm. the ventral diverticulum has separated from the 

 pharynx and forms a pear-shaped vesicle — ^the lateral thyroid vesicle — 

 lying free in the mesenchyme by the side of the trachea. Its lumen is 

 reduced to a mere slit — a result of the internal proliferation of its 

 walls. It bears the same relation to the th}Toid as in the pig of the 

 stage last described. 



I was unable to distinguish in this individual any clear evidence of 

 the presence of a "glandule thyroidienne." 



In a 20 mm. pig the lateral thyroid vesicle shows only fragments of 

 a lumen. Otherwise it is a solid structure and is composed of several 

 layers of small cells of epithelial nature, but closely resembling lymphoid 

 tissue. They have essentially the same character as the elements form- 

 ing the lobules of the thvmus. 



