6io 



PAIRED FINS OF ELASMOBRANCHII. 



by a line of columnar epiblast cells 1 . This connecting line of 

 columnar epiblast is a very transitory structure, and after its 

 disappearance the rudimentary fins become more prominent, 

 consisting (fig. 343, /;) of a projecting ridge both of epiblast and 

 mesoblast, at the outer edge of which is a fold of epiblast only, 

 which soon reaches considerable dimensions. At a later stage 

 the mesoblast penetrates into this fold and the fin becomes a 

 simple ridge of mesoblast, covered 

 by epiblast. The pectoral fins 

 are usually considerably ahead 

 of the pelvic fins in development. 



For the remaining history it 

 is necessary to confine ourselves 

 to Scylliurn as the only type 

 which has been adequately 

 studied. 



The direction of the original 

 ridge which connects the two fins 

 of each side is nearly though not 

 quite longitudinal, sloping some- 

 what obliquely downwards. It 

 thus comes about that the attach- 

 ment of each pair of limbs is 

 somewhat on a slant, and that 

 the pelvic pair nearly meet each 

 other in the median ventral line 

 a little way behind the anus. 



The elongated ridge, forming 

 the rudiment of each fin, gradually projects more and more, and 

 so becomes broader in proportion to its length, but at the same 

 time its actual attachment to the side of the body becomes 

 shortened from behind forwards, so that what was originally the 

 attacJied border becomes in part converted into the posterior 

 border. This process is much more completely carried out in 

 the case of the pectoral fins than in that of the pelvic, and the 

 changes of form undergone by the pectoral fin in its develop- 

 ment may be gathered from figs. 344 and 348. 



FIG. 343. .SECTION THROUGH 

 THE VENTRAL PART OF THE TRUNK 

 OF A YOUNG EMBRYO OF SCYLLIUM AT 

 THE LEVEL OF THE UMBILICAL CORD. 



/'. pectoral fin ; ao. dorsal aorta ; 

 cav. cardinal vein ; ita. vitelline ar- 

 tery; 7/.z>. vitelline vein; al. duode- 

 num ; /. liver ; sd. opening of seg- 

 mented duct into the body cavity ; 

 /;//. muscle plate ; inn. umbilical 

 canal. 



1 F. M. Balfour. Monograph on Elasmobranch Fi$lics, pp. 101 2. 



