730 MR. c. TATE KEGAN ON [June 19, 



development. But here we are met with the fact that Balfonr 

 independently arrived at the theory of the similar origin of 

 median and paired fins from their similar development in Selachian 

 fishes. 



In the types which he studied, Balfour found that the median 

 and paii'ed fins first appeared as special developments of continuous 

 ridges of columnar epiblast. In the case of the paired fins this 

 ridge was a very transitor}^ structure, connecting the fin-rudiments 

 only at their first development, and disappearing before they 

 became at all prominent. In the case of the unpaired fins the 

 connecting ridge attained a somewhat greater development before 

 disappearing. Balfonr considered that these facts could only bear 

 one iiiterpi-etation, viz., that the limbs were the i-emnants of 

 continuous lateral fins. 



But Prof. Graham Kerr gathers from Balfour that this ridge 

 connecting the paired fins does not occur in Scyllmm [Scylio- 

 rhinus), and considers that we now know that it is confined to 

 the Rays (Hypoti'emata), from which he infers that the continuity 

 of the paired fins in the embryo may only be a foreshadowing of 

 their extension along the sides of the body, which is so charac- 

 teristic of this group. 



Even if this ridge were confined to the Hypoti-emata, it must 

 be evident that a very transitory structure, connecting the fin- 

 rudiments only at their first development, can have no I'elation 

 to the secondary extension of the pectoral fins in these Selachian?. 

 But Balfour leaves no doubt as to what were the forms in which 

 he observed these phenomena. In the ' Monograph of the 

 Development of Elasmobranch Fishes,' p. 97, a footnote says: 

 " Unless the contrary is stated, the facts recorded in this cha,pter 

 [Chapter VI.] apply only to the genei^ Scyllmm and Frisimrtis." 

 After describing the early development of the fins, without 

 reference to special genera, he notes that the connection of 

 the paired fins is especially Avell shown in Torpedo. In his 

 wonderful text-book of 'Comparative Embryology,' p. 610, we 

 find : •' For the remaining histoiy it is necessary to confine 

 ourselves to Scyllmm 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, 

 etc., etc." 



In Balfour's account of the subsequent development the chief 

 interest attaches to the fin-skeleton. The piincipal points may 

 be stated thus : — The supports of both median and paired 

 fins are segmented from continuous laminae, the segmentation 

 being to a great extent completed before the difierentiation of 

 the tissue as cartilage. In Scyllium the fin-skeleton of both 

 pectoral and pelvic fins in its earliest stages consists of a bar 

 parallel to the long axis of the body, the outer side of which 

 is continued into a plate which extends into the fin and very 

 early becomes segmented into a series of parallel luys at right 



