[7 3 J THE EVOLUTIOlN[ OF THE FINS OF PISHES. lOf^S 



Xir, — On the influence of muscular metamerism on the de- 

 velopment OF the axial and appendicular skeleton of 



FISHES. 



Herbert Spencer,* in a discussion of the origin of the form of the 

 vertebrate skeleton, has, largely upon a priori grounds, attempted to 

 account for the origin of vertebral segments. The hypotheses which 

 he put forward were in the main justified b.y the facts then known, but 

 are not (juite in accord with the ontogenetic evidence since worked out 

 by a number of embryologists. He says (p. 201) : " It follows from the 

 mechanical hypothesis that as the muscular segmentation must begin 

 externally and progress inwards, so, too, must the vertebral segmenta- 

 tion." In defense of this view he instances the fact that the develop- 

 ment of the truly osseous skeleton is centripetal, or from without in- 

 wards, the peripheral parts ossifying first. He supijoses, in fact, that 

 vertebral segmentation is entirely due to the flexures of the body pro- 

 duced by the contractions of the lateral muscles of fish-like forms dur- 

 ing their movements through the water. 



Such an hypothesis cannot now be unqualifiedly accepted, since the 

 tendency amongst embryologists at the present time is to conclude that 

 the muscular segments are derived from a centrally-placed archenteron, 

 from which their rudiments grow out as hollow diverticula.! The 

 'effect of such a view, founded upon observed facts, as in the case of 

 Amp1iioxus,X upon the hypotheses suggested by Spencer, it will be 

 readily seen, is imjiortant, though it may be positively affirmed that 

 while considerable modification seems necessary, the essential elements 

 of- the great principle of the mechanical genesis of vertebral segments 

 suggested by the English philosopher remain true. The embryologists 

 show, in fact, that the evolution of the muscular system i>roceeds from 

 within outwards in the vertebrate embryo, instead of from without in- 

 wards, as is assumed by Spencer, though this does not impair the effi- 

 ciency of the lateral musculature and what other muscles are derived 

 from it in the fins in producing segmentation of axially-placed skeletal 

 structures consisting of cartilage, bone, and membrane. 



Sections cutting through the axis of a young Vertebrate, such as that 

 of the Catfish, for example, show that the fibrous septa between the 

 muscular segments end internally at the point where two successive 

 vertebrae articulate with each other. The points where the muscular 

 fibers begin and end therefore coincide with the points where segmenta- 

 tion of the skeletogenous tract, from which the vertebrae are developed, 



" Principles of Biology, II, Chap. XV, pp. 192-209. 



t Cy. Adam Sedgwick, Oa the origin of metameric segmentation and soiiu' otlier 

 morphological questions. Studies from the Morphological Laboratory of the Univer- 

 sity of Cambridge, II, Pt. 1, pp. 77-116. Also O. Hertwig, Die Ccelomtheorie, .Jena, 

 1881. 



X Cf. Stndien liber Entwickelung des Amphioxus, von B. Hatschek. Aib. auf* dem 

 7.00I. Inst. d. Uuiversitiit Wien, 1882. 



