76 Morphology of the Fins 



hand, we have a forward migration of the pectoral fin, which 

 gradually takes its place in advance of the hindmost gill-arches. 

 The accompanjdng cut is from Dean's paper, " Biometric 

 Evidence in the Problem of the Paired Limbs of the Verte- 

 brates" (American Naturalist for November, 1902). Dean con- 

 cludes that in Hcterodoutiis "there is no evidence that there 

 has ever been a migration of the fins in the Gegenbaurian sense." 

 "The gill region, at least in its outer part, shows no affinity 

 during proportional growth with the neighboring region of the 

 pectoral fin. In fact from an early stage onward, they are evi- 

 dently growing in opposite directions." 



Kerr's Theory of Modified External Gills. — "It is because 

 I feel that in the present state of our knowledge neither of the 

 two \dews I have mentioned has a claim to any higher rank 

 than that of extremely suggestive speculations that I venture 

 to sav a few words for the third view, which is avowedly a 

 mere speculation. 



"Before proceeding with it I should say that I assume the 

 serial homology of fore and hind limbs to be beyond dispute. 

 The great and deep-seated resemblances between them are such 

 as to. my mind seem not to be adequately explicable except on 

 this assumption. 



" In the Urodela (salamanders) the external gills are well- 

 known structures — serially arranged projections from the body- 

 wall near the upper ends of certain of the branchial arches. 

 When one considers the ontogenetic development of these 

 organs, from knob-like outgrowth from the outer face of the 

 branchial arch, covered with ectoderm and possessing a meso- 

 blastic core, and which frequently if not alwa^^s appear before 

 the branchial clefts are open, one cannot but conclude that 

 they are morphologically projections of the outer skin and 

 that they have nothing whatever to do with the gill-pouches 

 of the gut-wall. Amongst the Urodela one such gill projects 

 from each of the first three branchial arches. In Lcpidosiren 

 there is one on each of the branchial arches I-IV. In Polvptenis 

 and Calaiiioichtliys {ErpetoicJitJiys) there is one on the hyoid 

 arch. Finally, in many Urodelan larva; we have present at 

 the same time as the external gills a pair of curious structures 

 called balancers. At an early stage of my work on Lcpidosiren, 



