336 THE ORIGIN OF VERTEBRATES 
which the branchial basket-work stands in relief. If it were re- 
stored to its original condition of muco-cartilage, it would represent 
a uniform plate, on the wnder surface of which the basket-work 
would be situated; and if it were calcified and made solid, the 
branchial basket-work would not show at all in these figures. 
Is it possible to find the reason why this skeletal covering has 
degenerated so early before transformation, and why the thyroid 
plate remains intact until transformation? We see that all that part 
which has degenerated is covered over by the somatic muscles,—by, 
in fact, muscles which, being innervated by the foremost spinal 
nerves, belong naturally to the region immediately following the 
branchial. I suggest, therefore, that the original skeletal covering 
of muco-cartilage has remained intact only where it has not been 
invaded and covered over by somatic muscles, but has been invaded 
by blood and undergone the same kind of degenerative change as 
overtakes the great mass of this tissue at transformation wherever 
the somatic muscles have overgrown it. 
The covering somatic muscles in the branchial region form a 
dorsal and ventral group, of which the latter is formed in the embryo 
much later than the former, the line of separation between the two 
groups being the lateral groove, with its row of branchial openings. 
This groove ends at the first branchial opening, but the ventral and 
dorsal somatic muscles continue further headwards. It is instruc- 
tive to see that, although the lateral groove terminates, the separation 
between the two groups of muscles is still marked out by a ridge 
of muco-cartilage, represented in Fig. 134, A, which terminates 
anteriorly in the opercular bar. 
Passing now to the prosomatic region, we find that here, too, the 
muco-cartilaginous external covering is divisible into a dorsal and 
a ventral head-plate, the ventral head-plate being the plate of the 
lower lip, and the dorsal head-plate the plate of muco-cartilage 
over the front part of the head. The staining reaction with thionin 
maps out this dorsal head-plate in a most beautiful manner, and 
shows that the whole of the upper lip-region in front of the nasal 
orifice is one large plate of muco-cartilage, obscured largely by the 
invasion of the crossing muscles of the upper lip, but left pure and 
uninvaded all around the nasal orifice, and where the upper and lower 
lips come together. In addition to this foremost plate, a median 
tongue of muco-cartilage covers over the pineal eye and fills up the 
