( 816 ) 



fin. In this fin the myotomes are pressed together so closely that a 

 direct counting of the number composing the fin is no more possible ; 

 by comparing' the total number of vertebrae to the number of 

 myotomes composing the first and second dorsal fins, we find that 

 about 70 myotomes give an element to the dorsal part of the 

 caudal fin. 



The results obtained by this indirect method are corroborated by 

 the result of the direct dissection. If we take a myotome giving a 

 muscular element upon the more anterior part of the dorsal fin and 

 begin the dissection with lamella b in the neighbourhood of the 

 second peak and proceed preparing caudally, we find lamella b being 

 rolled in, towards the mesial plane of the body, in the shallow 

 excavation in which the base of the fin rests, (tig. VI}. Along this way 

 the muscular tissue becomes gradually atrophic and only a thin band 

 remains, consisting of the connective tissue which forms the frame- 

 work for the muscle fibres. In the neighbourhood of the sagittal 

 plane of the body this lamella is folded, in such a manner, that the 

 line of folding (figure VI L" L") runs parallel to the sagittal axis of 

 the body. By this process of infolding the direction of the lamella b 

 is reversed, the infolded part proceeding cranially; this part of 

 lamella b passes into the dense sheath of connective tissue, which 

 is interposed between the dorsal musculature and the base of the 

 fin. As far as I can see this sheath of connective tissue is chiefly 

 built up by a large number of these lamellae, but they are so inextri- 

 cably united that I have not been able to follow lamella b in this 

 sheath. If starting from the fin, we prepare free one muscular ele- 

 ment of the fin, and this element is lifted up with enough precau- 

 tion, it may be seen, that from the base of such a fin-element as well 

 a thin lamella of connective tissue passes into that sheath of tissue in 

 which we could follow the reversed part of lamella b. The direct 

 continuity however of both lamellae in the sheath of dense connec- 

 tive tissue, I have not been able to establish. 



The muscular elements composing the fin (figure VI) are trian- 

 gular laminae; one side of the triangle is contiguous to the fin-rays 

 and the connective tissue which unites these rays in the mesial plane 

 of the body, the lateral side forms part of the lateral surface of 

 the fin, while the base is excavated and moulded upon the shallow 

 depression in the dorsal musculature. From the outside a septum of 

 intermyotomal tissue (s. i. figure VI) penetrates into the muscular 

 substance of the fin dividing this substance into & lateral (b) and a 

 mesial part (a). This septum inserts a little above the muscular 

 substance upon the fin-rays, and becomes thinner and thinner without 



