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36 THE ORIGIN OF VERTEBRATES 



which the branchial basket-work stands in relief. If it were re- 

 stored to its original condition of nmco-cartilage, it would represent 

 a uniform plate, on the under 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 



