ORIGIN OF THE FINS OF FISHES. 545 



nature of the paired fins, and upon what an exceedingly slender basis 

 rest both of the two views which at present hold the field?" 



Kerr's Theory of Modified External Gills. 



"It is because I feel that in the present state of our knowledge 

 neither of the two views I have mentioned has a claim to any higher 

 rank than that of extremely suggestive speculations that I venture to 

 say 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 possess- 

 ing a mesoblastic core, and which frequently if not always 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 Lepidosiren there is one on each of branchial arches I .-IV. 

 In Polypterus and Calamoichthys (Erpetoichthys) there is one on the 

 hyoid arch. Finally, in many Urodelan larvae we have present at the 

 same time as the external gills a pair of curious structures called bal- 

 ancers. At an early stage of my work on Lepidosiren, while looking 

 over other vertebrate embryos and larvae for purposes of comparison, 

 my attention was arrested by these structures, and further examinations 

 by section and otherwise, convinced me that they were serial homologues 

 of the external gills, situated on the mandibular arch. On then looking 

 up the literature, I found that I was by no means first in this view. 

 Eusconi had long ago noticed the resemblance, and in more recent times 

 both Orr and Maurer had been led to the same conclusion as I had been. 

 Three different observers having been independently led to exactly the 

 same conclusions, we may, I think, fairly enough regard the view I have 

 mentioned of the morphological nature of the balancers as probably a 

 correct one. 



"Here then, we have a series of homologous structures projecting 

 from each of the series of visceral arches. They crop up on the Cross- 

 opterygii, the Dipnoi and the Urodela, i. e., in three of the most archaic 

 of the groups of Gnathatomata. But we may put it in another way. 

 The groups in which they do not occur are those whose young possess a 



VOL. LXI. — 35. 



