THE ORIGIN OF VERTEBRATE LIMBS 429 



into contact. The connection must, therefore, be interpreted 

 as secondary. 



(3) The pectoral girdle of Cestracion as it develops moves 

 toward the gill region. When it first appears, the scapular por- 

 tion of the girdle is separated from the last gill arch by a con- 

 siderable space, but as development proceeds the girdle and the 

 arch approach each other until the intervening space is elimi- 

 nated. At first this space is fully twice as great as that between 

 the gill arches; at 35 mm. it is half passed over, and at 60 mm 

 the arch and girdle are practically in contact. In the adult 

 they overlap slightly. 



There is, moreover, no evidence that gill arches may be 

 crowded or pushed out of the branchial region. It has, on the 

 other hand, been proved that the sixth gill of Cestracion de- 

 generated in situ, if the structures which Mrs. Hawkes . ('°5) 

 describes are to be interpreted as the vestiges of a gill arch. 



IV. First, as to time relations, it is important to note that 

 all the other structures of the fin make their appearance in ad- 

 vance of the skeleton, and before the gill arches are differentiated. 

 On the assumption that the fin has been formed out of a displaced 

 gill, we should expect to find the skeleton developing in an 

 outward direction and carrying with it the other structures 

 which form the fin. But instead of this, the fin fold, with its 

 muscle-buds, nerves, and blood-vessels, as well as the primitive 

 support of the fin (the mesenchyme thickening) , is well developed 

 before the skeleton becomes evident. Moreover, the fin skeleton 

 does not grow into the fin fold, for there is no disarrangement 

 or shifting of parts as the skeleton appears, but the skeleton 

 forms in situ by differentiation of the original mesenchyme 

 support. This is just as it should be on the fin-fold theory, 

 but exactly opposite to what would be expected on the gill-arch 

 theory, if time relations stand for anything. The order of ap- 

 pearance, furthermore, is precisely as it is in the unpaired fins. 



Second, with regard to place relations, we have already shown 

 that the first rudiment of the pectoral arch is more ventral than 

 that of the gill arch, that it is more external, and that in Ces- 

 tracion at least it grows toward the gill region as it 

 develops. 



