36 Mr. K. G. Harrison on the 



nuclei which will ultimately belong to muscle and those of 

 the connective tissue. 



By this time cartilaginous rods, alternating with the 

 muscle-pairs, have appeared in the middle plane of the fin. 

 These become the interspinal bones which support the rays 

 of the dorsal fin ; in the anal fin they are the interheemals. 

 The chondrification takes place centripetally. I shall call 

 them ray-supports. 



Horizontal sections of this stage show clearly the serial 

 arrangement of the various structures of the fin. Opposite 

 the cartilaginous rods the ectoderm is constricted to a marked 

 degree. At these constrictions mesenchyme cells have aggre- 

 gated close to the ectoderm, forming loose strands of cells, 

 one on each side of each ray-support. This is the beginning 

 of the definitive depressor muscles of the fin-rays. 



The dermal rays now begin to develop, in lines which are 

 distally continuous with each erector muscle rudiment, the 

 cells of which are not separated from those of the corre- 

 sponding rays by any sharp dividing line. Considerably 

 later a small nodule of cartilage is formed at the tip of each 

 cartilaginous ray-support, which has in the meantime become 

 considerably bent with its convexity forward. Each pair of 

 dermal rays grasps with its basal end the corresponding 

 cartilaginous ball, and a strong fibrous tissue binds them 

 together. Muscles now become inserted into this mass, in 

 such a manner that each ray receives one pair of each of the 

 three muscles belonging to each segment of the fin. Anterior 

 to the pivot on which the cartilaginous ball rests the erector 

 is attached ; posterior to the pivot the depressor and the 

 muscle which takes origin from the skin. The depressor and 

 the erector arise from the ray-supports. The fin has now 

 practically reached its adult condition, except that the carti- 

 laginous skeleton has not yet ossified. 



The above account holds good only for those segments 

 which do not lie at the ends of the fin. In the first two or 

 three segments the course of development is considerably 

 modified, although the same definitive arrangement is reached. 

 The number of myotomes which produce buds is variable, 

 but, as a rule, ten or eleven reach the dorsal fin and eight the 

 anal. Both anterior and posterior to these buds may be 

 formed, but they do not reach more than rudimentary deve- 

 lopment. The number of these buds is very variable. When 

 the cartilage has just begun to appear, and the muscle-masses 

 are on their way to segregation trom the surrounding mesen- 

 chyme, these rudimentary buds have disappeared, presumably 

 having disintegrated into ordinary mesenchyme tissue. Both 



