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501 



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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 con- 

 siderably ahead of the pelvic fins in 

 development. 



For the remaining history it is 

 necessary to confine ourselves to 

 Scyllium 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 longi- 

 tudinal, sloping somewhat obliquely 

 downwards. It thus comes about 

 that the attachment 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 pro- 

 jects more and more, and so becomes 

 broader in proportion to its length, 

 but at the same time its actual at- 

 tachment to the side of the body 



becomes shortened from behind forwards, so that what was originally 

 the attached 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 development may be gathered 

 from figs. 344 and 348. 



Before proceeding to the development of the skeleton of the fin 

 it may be pointed out that the connection of the two rudimentary 

 fins by a continuous epithelial line suggests the hypothesis that they 

 are the remnants of two continuous lateral fins 1 . 



Shortly after the view that the paired fins were remnants of con- 

 tinuous lateral fins had been put forward in my memoir on Elasmo- 

 branch Fishes, two very interesting papers were published by Thacker 

 (No. 489) and Mivart (No. 484) advocating this view on the entirely 

 independent grounds of the adult structure of the skeleton of the 

 paired fins in comparison Avith that of the unpaired fins 2 . 



1 Both Maclise and Humphry (Journal of Anat. and Phijs., Vol. v.) had previously 

 suggested that the paired fins were related to the unpaired fins. 



2 Davidoff in a Memoir (No. 477) which forms an important contribution to our 

 knowledge of the structure of the pelvic fins has attempted from his observations to 

 deduce certain arguments against the lateral fin theory of the limbs. His main 



ft m 



FIG. 343. SECTION THROUGH THE 

 VENTRAL PAET OF THE TRUNK OF A 

 YOUNG EMBRYO OF SCYLLIUM AT THE 

 LEVEL OF THE UMBILICAL CORD. 



b. pectoral fin ; ao. dorsal aorta ; 

 cav. cardinal vein ; ua. vitelliue ar- 

 tery ; uv. vitelline vein ; al. duode- 

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

 mented duct into the body cavity; 

 mp. muscle plate ; urn. umbilical 

 canal. 



