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 
> 
queues” 
SS 
SaGeeES 
aaa +X 
A B 
Fig. 84.—D1acram or (A) VENTRAL SURFACE AND (B) LaTERat Surrace or AMMO- 
C@TES, SHOWING THE ARRANGEMENT OF THE EPITHELIAL Pits on THE BRAN- 
cHIAL REGION, AND THEIR INNERVATION By VII., Tue Factan, [X., 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 
