I conclude, therefore, that originally the branchial or mesosomatic region was covered with a dorsal plate of muco-cartilage, which carried on its under surface the dorsal part of the branchial basket-work, and sprang from the central core of skeletogenous tissue around the notochord; this plate was separated from the plate which covered this region ventrally by the lateral grove in which the gill-slits are situated. The ventral plate carried on its under surface the ventral part of the branchial basket-work, and was originally continuous with the plate over the thyroid gland. {335}[Illustration: FIG. 134.--SKELETON OF HEAD-REGION OF AMMOCOETES. A, LATERAL VIEW; B, VENTRAL VIEW; C, DORSAL VIEW. Muco-cartilage, _red_; soft cartilage, _blue_; hard cartilage, _purple_. _sk_1_, _sk_2_, _sk_3_, skeletal bars; _c.e._, position of pineal eye; _na. cart._, nasal cartilage; _ped._, pedicle; _cr._, cranium; _nc._, notochord.] {336}In Fig. 134, A, B, C, the cranial skeleton of Ammocoetes is represented from the dorsal, ventral, and lateral aspects. The muco-cartilage is coloured red, the branchial or soft cartilage blue, and the hard cartilage purple. The degenerated muco-cartilage of the branchial region is represented as an uncoloured plate, on which the branchial basket-work stands in relief. If it were restored to its original condition of muco-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 instructive 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.