( 818 ) 



to the ventral musculature (lam. ƒ figure II and fig. VII) becomes 

 shorter and the first peak of the ventral musculature directed crani- 

 al ly (peak 7, figure II), more and more develops into a true peak. 

 Now by the disappearence of lamella ƒ peak 6 and 7 approach each 

 other, remaining divided, however, by a thin lamella of connective 

 tissue penetrating into the second fold alqng the line L'L' (fig. VIII 

 and IX). In consequence of the process of infolding peak 6 lies mesially 

 in respect to peak 7 which covers peak 6 from the outside. At the 

 level of myotome 15 (reckoned from the first myotome, giving an 

 muscular element to the first dorsal fin) the second fold vanishes. 

 Together with the disappearence of the fold we notice the vanishing 

 of the displacement of the lateral musculature in respect to the 

 ventral musculature, which was only a consequence of the process 

 of infolding, so that the two peaks (6 and 7) lie side by side in the 

 same transversal level of the body. At the place of disappearance 

 of the second fold the two peaks unite to a single peak directed 

 cranially. Together with the disappearance of lamella ƒ we notice 

 the further development of lamella </. 



At the same level where the second lateral fold disappears, we 

 find the appearance of the cartilagineous plate, uniting the two 

 basipterygii of the pelvic fins. With its front border, this plate folds 

 in lamella g (figure II and X) from the inside so that this lamella 

 covers the front e<\ ( ^e of this plate; in this way the pelvic fin is 

 formed. The details of the formation of the pelvic fins I have not 

 yet investigated. By the formation of the pelvic fin peak 8 (fig. II)- 

 and lamellae g and h pas> into the musculature of the fin, so that 

 in a transverse section through the animal, at the level of the pelvic 

 fin, the trunkmusculature is only composed by five peaks (viz. 2. 

 3. 4. 5. (6 -J- 7) of figure II). This structure of the transverse 

 sections does not change any more proceeding along the body 

 caudally (figure XI and XII). 



The disappearance of the first fold, dividing the dorsal from the 

 lateral musculature, takes place in the same way as described for 

 the second fold, at the level of myotome 45 (reckoned from the 

 first myotome giving an element to the first dorsal fin); only the 

 case is more simple not being complicated by the presence of two peaks. 



Finally I paid attention to the influence of the abdominal eavity 

 upon the form of the myotome. I found this influence to be very 

 restricted, as it only determines to some extent the dimensions of' the 

 myotome, without producing any particular differentiation in its form. 



In fine I wish to express my thank to Prof. Langelaan for his 

 aid and assistance in these researches. 



