The chief observations described above may be summarised as follows : The development of the median dorsal fins is essentially similar to that of the paired fins. They arise as longitudinal folds, into which grow buds from the myotomes. Some fourteen or sixteen myotomes contribute to the fin each one muscle-bud. Concentration sets in almost from the first appearance of the buds; it is chiefly, if not entirely, due to the body growing faster than the fin. Along the greater part of the dorsal fin each muscle-bud becomes converted into one radial muscle. At the extreme ends of the fins the exact metameric origin of the muscles is difficult to trace and is somewhat obscured. Only here fusion of neighbouring segmental buds perhaps takes place. At certain stages slender strands of embryonic tissue connect the bases of the radial muscles; these are probably rudiments of the nerve-plexus. Neither the study of development nor of the adult structure affords any definite evidence that the primitive metamerism of the musculature is lost. Experiments seem to establish that the radial muscles remain haploneurous, retaining their primitive connection with the nerve belonging to that myotome from which they have been developed. The nerve- "plexus" of the fins is composed of intertwining sensory fibres, along or through which the motor fibres proceed to their destination without mixing with those of other segments. There is The chief observations described above may be summarised as follows : The development of the median dorsal fins is essentially similar to that of the paired fins. They arise as longitudinal folds, into which grow buds from the myotomes. Some fourteen or sixteen myotomes contribute to the fin each one muscle-bud. Concentration sets in almost from the first appearance of the buds; it is chiefly, if not entirely, due to the body growing faster than the fin. Along the greater part of the dorsal fin each muscle-bud becomes converted into one radial muscle. At the extreme ends of the fins the exact metameric origin of the muscles is difficult to trace and is somewhat obscured. Only here fusion of neighbouring segmental buds perhaps takes place. At certain stages slender strands of embryonic tissue connect the bases of the radial muscles; these are probably rudiments of the nerve-plexus. Neither the study of development nor of the adult structure affords any definite evidence that the primitive metamerism of the musculature is lost. Experiments seem to establish that the radial muscles remain haploneurous, retaining their primitive connection with the nerve belonging to that myotome from which they have been developed. The nerve- "plexus" of the fins is composed of intertwining sensory fibres, along or through which the motor fibres proceed to their destination without mixing with those of other segments. There is probably no real motor plexus, but the motor nerves may be gathered together into more or less longitudinal collectors, and become again sorted out on reaching the musculature. Such collectors are found at the base of the dorsal fins, compounded of some fourteen to sixteen segmental rami pterygiales. All the fins remain throughout development in approximately the same position. Apparent change of place may be brought about by concentration being greater in the one direction than in the other. This is especially the case with the dorsal fins, the anterior edge of which may undergo a relative shifting over some ten segments. The general bearing of these results has been sufficiently discussed in the Introduction (p. 334), and need not again be dealt with here. But it may be pointed out how completely they support the lateral fold theory of the origin of the paired fins.