200 



THE ORIGIN OF VERTEBRATES 



The evidence that such a median segment has been interpolated 

 ventrally between the foremost pairs of branchial segments is 

 remarkably clear, for the limits ventrally of the branchial segments 

 are marked out on each side by the ventral border of the carti- 

 laginous basket-work ; and it is well known, as seen in Fig. 80, that 

 whereas this cartilaginous framework on the two sides meets together 

 in the middle ventral line in the posterior branchial region, it diverges 

 in the anterior region so as to form a tongue-shaped space between 



-. IX 



. X J 



X 2 

 X 3 



4-u.LatVII + X 



Fig. 84.— Diagram of (A) Ventral Surface and (B) Lateral Surface of Ammo- 



C03TES, SHOWING THE ARRANGEMENT OF THE EPITHELIAL PlTS ON THE BRAN- 

 CHIAL Region, and their innervation by VII., the Facial, IX., the 

 Glossopharyngeal, and X'-X", the Vagus Nerves. 



the branchial segments on the two sides. This space is covered over 

 with a plate of muco-cartilage which bears on its inner surface the 

 thyroid gland. 



In addition to this evidence that we are dealing here with a 

 ventral tongue-like segment belonging to the facial nerve which is 

 interpolated between the foremost branchial segments, we find the 

 most striking fact that at transformation the whole of this muco- 

 cartilaginous plate disappears, the remarkable thyroid gland of the 



