86 'COMPARATIVE ANATOMY. 



In many tailed Amphibians and Amphibian larvse (including 

 those of Csecilians), the unpaired fins are represented by a fold of 

 the skin extending along the dorsal and ventral sides of the tail. 

 In some cases this fold extends along the back right up to the 

 head, but it never gives rise to bony or cartilaginous supporting 

 elements. In the male Triton it becomes much enlarged during the 

 breeding season. 



b. PAIRED LIMBS. 



No other morphological problem has given rise during the last 

 twenty years to such extensive researches and to such varied 

 solutions as the question of the origin of the paired limbs. Two 

 very opposite views exist. According to one of these (Gegenbaur's 

 view), the proximal parts of the extremities, that is, the pectoral 

 and pelvic arches, are regarded as being derived from branchial 

 arches, and the distal or free portions as metamorphosed 

 fin-rays. According to this theory, the pelvis is to be looked 

 upon as a visceral arch which has changed its position so as to 

 lie far back along the body. 



According to the other view (that of Dohrn), the origin of the 

 paired limbs has nothing to do with the visceral skeleton, but, like 

 the latter, they are to be looked upon as the localised remains in 

 definite regions of the body (thoracic and pelvic regions) of a series 

 of cartilaginous bars originally extending along the whole trunk, 

 and having a metameric arrangement. In other words, just as 

 each body-segment of an Annulate may be looked upon as being 

 provided with a pair of limbs, so also was each primitive segment 

 of the Vertebrate body : recent researches seem to support this. 



These researches were made on Elasmobranch embryos, and in these each 

 somite gives rise to a fin-element, each of which consists of two dorsal and two 

 ventral bundles of muscle, two rods of cartilage, arid a corresponding spinal 

 nerve. Both pectoral and pelvic fins are made up of a considerable number 

 of these fin-elements. It is interesting to note that these outgrowths from 

 the somites are present along the entire length of the lateral epiblastie. 

 folds, that is, they exist at first between the pectoral and pelvic fins, as well 

 as behind the latter, but in these regions they soon become aborted, The 

 lateral epiblastic folds do not run parallel to one another, as was supposed by 

 Thatcher, Mivart, and Balfour, but converge towards the anus (Fig. 08, A) : the 

 presence of outgrowths behind the anus, however, points to the possibility of the 

 ventral unpaired fin having been originally paired. This probably was the ease 

 when the postanal gut of the embryo, as well as the coelonie, extended through 

 the whole caudal region. After the formation of the definitive amis and the 

 disappearance of the postanal gut, the two lateral halves of each primordial 

 fin-fold fused together to form a median fin. Possibly the dorsal fin was also 

 originally paired, for it arises by paired outgrowths from the dorsal part of 

 the myotomes, so that in this case it was not situated in the middle line, but 

 along each side of it. 1 On the closure of the laminae dorsales in the formation 



1 Dohrn lias lately attempted to prove that the unpaired lins of Petromyzon arise 

 in a paired manner, and that this Fish must fornnTly have possessed paired pectoral 

 ami pelvic lins, which have gradually become lost. 



