Development of the Fins of TeJeosts. 35 



definitive muscles of the elasmobranch fins are known to 

 develop. The following observations made upon the salmon 

 {Salmo solar) render a modification of this view necessary. 



The Unpaired Fins. — The unpaired fins arise as prolifera- 

 tions of the mesenchyme cells, which, in the form of a loose 

 meshwork, fill the median fin-folds. The caudal fin is the 

 first to appear, and is followed by the dorsal, the anal, and 

 the adipose, in the order named. 



Shortly after the dorsal thickening has appeared muscle- 

 buds appear at the anterior dorsal angles of the myotomes of 

 that region, and grow rapidly into the fin-rudiment, as has 

 been described by Dohrn (" Studien IX.," ' Mittheilungen 

 aus der Zool. Stat, zu Neapel,' Bd. vi.). These processes 

 converge considerably towards one another, so that, while the 

 middle ones project at right angles to the long axis of the 

 body of the fish, those at each end of the fin cut the axis at 

 an angle of about forty-five degrees. Cross sections show 

 that the buds are solid ; a few cells are enclosed by an 

 epithelium of similar cells. The nuclei closely resemble 

 those of the mesenchyme, and the cell boundaries are indistinct. 

 The buds are continuous with the cells of the lateral layer of 

 the myotomes (cutis-plate). Similar buds grow out from the 

 anterior ventral angle of the myotomes in the region of the 

 anal fin. The tail-fin also receives outgrowths from several 

 of the terminal myotomes ; the adipose fin never contains 

 muscle-buds or muscle-tissue of any kind. As the buds grow 

 further into the fin-rudiment their outer ends become enlarged 

 and somewhat flattened against the epidermis, in which a 

 considerable bulging is caused. The stalk now disintegrates, 

 and its component cells can no longer be distinguished from 

 the mesenchyme. In the meantime the nuclei which lie in 

 the median half of each bud accumulate considerable cyto- 

 plasm as a first step towards difterentiation into muscle-cells. 

 These masses of embryonic muscle-cells now grow centri- 

 petally ; they ultimately become the erector muscles of the 

 fin-rays. The lateral half of the bud now loses its identity 

 as a cell-mass, having become undistinguishable from mesen- 

 chyme; but very soon the cells which lie opposite the peri- 

 pheral end of each bud accumulate more cytoplasm, and these 

 masses also grow towards the body, remaining close to the 

 epidermis. They ultimately become the superficial muscles 

 of the rays, which in the adult take origin from the skin. 

 By this time the mesenchyme has developed to such an 

 extent that the muscle-masses are not at all clearly defined, 

 so that it is impossible to draw a sharp dividing line between 



3* 



