Morphology of the Fins 77 



while looking over other vertebrate embryos and larvae for pur- 

 poses of comparison, my attention was arrested by these struc- 

 tures, and further examinations, by section or otherwise, convinced 

 me that there 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. Rusconi 

 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 inde- 

 pendently 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 pro- 

 jecting from each of the series of visceral arches. They crop 

 up on the Crossopterygii, the Dipnoi, and the Urodela, i.e., in 

 three of the most archaic of the groups of Gnathostomata. But 

 we may put it in another way. The groups in which they do 

 not occur are those whose young possess a very large yolk-sac 

 (or which are admittedly derived from such forms). Now 

 wherever we have a large yolk-sac we have developed on its 

 surface a rich network of blood-vessels for purposes of nutrition. 

 But such a network must necessarily act as an extraordinarily 

 efficient organ of respiration, and did we not know the facts we 

 might venture to prophesy that in forms possessing it any other 

 small skin-organ of respiration would tend to disappear. 



"No doubt these external gills are absent also in a few of 

 the admittedly primitive forms such as, e.g., (Neo-} Ceratodus. 

 But I would ask that in this connection one should bear in 

 mind one of the marked characteristics of external gills their 

 great regenerative power. This involves their being extremely 

 liable to injury and consequently a source of danger to their 

 possessor. Their absence, therefore, in certain cases may well 

 have been due to natural selection. On the other hand, the 

 presence in so many lowly forms of these organs, the general 

 close similarity in structure that runs through them in different 

 forms, and the exact correspondence in their position and rela- 

 tions to the body can, it seems to me, only be adequately ex- 

 plained by looking on them as being homologous structures 



