( 815 ) 



1 and the following myotoms by subsequent numbers. Alter that, 

 transverse sections of the animal were made, of 1 — 2 cm. thickness, 

 and the numbering transferred to these sections, so that the lamellae 

 belonging to one and the same myotome received the same number. 

 For the sake of an easy description, figure II gives a hem i schematic 

 representation of the myotome, in which the peaks are indicated by 

 numbers, the lamellae by letters. In figure III which is the trans- 

 verse section, (indicated in figure I with an A) the peaks appear 

 as systems of concentric lamellae marked in accordance with the 

 marking in figure II. If we now pass to the region of the first 

 dorsal fin (figure IV, section B of figure I and fig. V, section 6' of fig. I; 

 the image of the transverse section is changed, instead of being com- 

 posed by four peaks, the dorsal musculature only shows three peaks. 



The peak, indicated as number 1, has disappeared and instead 

 of this peak we find the first lamellae of the dorsal fin. Now in all 

 subsequent sections this first peak does not reappear any more. By 

 the method of successsive numbering it was possible to determine 

 the first myotome losing its most dorsal peak (number 1). The 

 external surface of this myotome is blackened in figure I. From the 

 principle laid down in the beginning of this notice ensues, thai the 

 myotome apparently losing its first peak, gives a muscular element 

 to the dorsal fin; this element is therefore also blackened in figure I. 

 It may be seen in figure I that the first myotome giving an element 

 to the fin lies a little caudally in respect to the front edge of' this 

 fin. The number of myotomes giving a part to the first dorsal tin 

 may easily be determined, because these composing parts of' the fin 

 are separated by intersegmental tissue; that we have really to deal 

 with intersegmental tissue follows from the fact that through these 

 lamellae bloodvessels and extremely fine nerve fibres reach the skin 

 (vid. v. Bisselick "On the Innervation of the Trunk-myotome"). The 

 total number of muscular elements composing the fin, amounted to 

 34, so that the last myotome still giving an element to the first 

 dorsal fin already lies in the region of* the second dorsal fin. From 

 the fact that the most dorsal peak (number 1) does not reappear 

 any more in the transverse sections, it follows that the next myotome 

 gives the first element to the second dorsal fin. The surface of this 

 myotome is also blackened in figure I to show its position in relation 

 to the front edge of the second dorsal fin. If is evident, that this 

 myotome occupies the same position in respect to the second dorsal 

 fin as the first myotome does in respect to the first dorsal fin. The 

 number of myotomes composing the second dorsal fin amounts to 30. 



Upon the second dorsal fin follows the dorsal part of the caudal 



56 



Proceedings Royal Acad. Amsterdam. Vol. IX. 



