( 817 ) 



reaching t ho base of the fin. At the base therefore the lateral 



and mesial parts of the muscular substance are continuous and form 

 a peak (figure VI p. 1), lying quite near the mesial septum of the 

 body. This peak must therefore represent the peak which is lost in 

 the transverse section (figure IV) made at the level of the first dorsal 

 fin. The septum penetrating into the muscular substance of the 

 fin is therefore the intermyotomal septum stretched out between 

 lamellae a and b of figure II. 



It ensues therefore from the combined observations, that the first 

 dorsal fin (and the same applies to the other dorsal tins) is differen- 

 tiated by a process of infolding similar to the differentiation between the 

 dorsal and the lateral and between the lateral and the ventral mus- 

 culature. The line of infolding crosses lamella b. In that part of the 

 lamella, which lies in the depth of the fold the muscular tissue is 

 atrophic. Proceeding from peak 2 caudally along lamella b the atrophy 

 of the musclefibres gradually increases, whilst on the other hand 

 proceeding from peak 1 caudally along lamella b (as far as it lies upon 

 the fin) the atrophy of the muscular tissue is abrupt. The position of 

 peak 1 has not changed in respect to the mesial plane of the body, 

 only the lamellae have changed their direction. The superior cornu 

 (i. e. lamella a) is no longer directed cranially but turned upwards 

 and this is also the case with that part of lamella b that has passed 

 into the fin. In connection with this representation of the facts, I del <t- 

 mined the direction of the muscle fibres in the fin ; here they slope 

 downwards from the intermyotomal septum. Now if we imagine the 

 lamellae composing the fin restored to their original position, the 

 course of the fibres in lamella a would be from mesial to lateral 

 and from caudal to cranial and this was actually the direction of 

 the muscle fibres in lamella <t in the region cranially of the first 

 dorsal fin. 



Differentiation of the latero-ventral musculature. 



The lateral musculature, as described by van Bisselick, shows a 

 peak directed caudally (peak 5, figure II and fig. VIIj situated near I he 

 second line of infolding L'L'. Proceeding along the body a second 

 peak appears directed cranially. The first myotome showing this peak 

 (peak 6, fig. II and fig. VII), is the eleventh myotome following the first 

 myotome giving a muscular element to the first dorsal fin. The two 

 peaks lie near to each other in the neighbourhood of the second 

 fold. In consequence of the infolding of the myotome at that place, 

 they do not reach the surface of the body, being covered from the 

 outside by the ventral musculature. Meanwhile the ventral part of 

 the myotome undergoes a change in form, the first lamella belonging 



56* 



