vii EVOLUTIONARY ORIGIN OF LIMBS 445 



short sketch of each and of the arguments for and against it may 

 now be given. The two hypotheses indicated are the " Lateral Told " 

 hypothesis of Balfour, Mivart, Thacher and others and the " Gill- 

 septum " hypothesis of Gegenbaur and his school.. Each hypothesis 

 concerns itself with the origin of the paired limbs of fishes — the 

 fin being regarded as the primitive type of limb from which the 

 pentadactyle limb has been evolved later on. 



The Lateral-fold Hypothesis.— It will be remembered that 

 the mode of development of the median unpaired fins indicates 

 clearly that these fins are simply persisting and exaggerated portions 

 of a once continuous median fin-fold. According to the lateral-fold 

 hypothesis of the origin of the limbs the paired fins of fishes are 

 similarly to be looked on as persisting and enlarged portions of a 

 continuous fin-fold which once extended along each side of the body. 

 The hypothesis rests upon a tripod basis of Embryological, Ana- 

 tomical and Palaeontological fact. 



Balfour in his Development of Elasmobranch fishes (1878) wrote : 

 " Along each side of the body there appears during this stage (G-I) 

 a thickened line of epiblast, which from the first exhibits two 

 special developments : one of these just in front of the anus, and a 

 second and better marked one opposite the front end of the 

 segmental l duct. These two special thickenings are the rudiments 

 of the paired fins, which thus arise as special developments of a 

 continuous ridge on each side, precisely like the ridges of epiblast 

 which form the rudiments of the unpaired fins." " If the account 

 just given of the development of the limbs is an accurate record of 

 what really takes place, it is not possible to deny that some light is 

 thrown by it on the first origin of the vertebrate limbs. The facts 

 can only bear one interpretation, viz. that the limls are the remnants 

 of continuous lateral fins." 



Further embryological support to this hypothesis has been pro- 

 vided (1) by the fact that the muscles of the limb are of segmental 

 origin, derived from a number, often a considerable number, of 

 myotomes (see p. 207) and that apparently vestigial muscle-buds 

 have been found both headward and tailward of the series which 

 take part in the muscularization of the definitive fin, and (2) by the 

 fact that the fin type of limb commonly shows a marked narrowing 

 of its base of attachment during the- process of development, the 

 rudiment having during early stages the form of a longitudinal ridge 

 attached throughout its length to the body. 



Regarding the evidence upon which the fin-fold hypothesis rests 

 the following criticisms may be expressed. (1) The ectodermal ridge 

 described as connecting the two limb rudiments in Elasmobranchs 

 turns out to be a characteristic not of Elasmobranchs in general but 

 only of Rays (Torpedo) i.e. of forms in which there is an enormous, 

 admittedly secondary, extension of the pectoral fin along the side of 

 the body. The Elasmobranchs less specialized in this respect— the 



1 = Arehinephrio. 



