174 EMBRYOLOGY OF THE LOWER VERTEBRATES oh. 



in the development of this asymmetry are seen in Polypterus and in 

 the Lung-fishes. 



41 In purely aquatic creatures the dictates of adaptation would 

 naturally cause the air-filled lung to assume a dorsal position. An 

 initial phase of this is repeated in Polypterus where the right lung 

 has become dorsal and median in its hinder portion. In the Lung- 

 fishes a further step is taken — the whole of the lung becoming dorsal 

 except the pneumatic duet which still remains to mark out the path 

 by which the lung moved dorsalwards round the right side of the 

 alimentary canal. 



That the movement dorsalwards was round the right side was no 

 doubt due to the right lung being predominant and the left reduced 

 in size. In the case of Ceratodus the predominance of the original 

 right lung has been retained, the other being completely obsolete 

 except for a, short period during development. In Zepidosiren and 

 Protopterus, on the other hand, the lopsidedness disappears, the 

 original left lung regaining during ontogeny its primitive equality in 

 size with its fellow. 



5. In the Actinopterygians — those fishes which show the highest 

 degree of evolution in adaptation to a swimming mode of life — the 

 lung has in the course of its evolution passed through similar stages 

 to those exemplified by Polypterus and Ceratodus. Here again only 

 the original right lung persists as the air-bladder, the vestige of the 

 left lung being possibly represented by the little diverticulum found 

 by Moser upon the pneumatic duct in early stages of development. 1 

 In the Actinopterygians a further step' onwards has been made in 

 that the glottis has assumed a dorsal position. This is fully ex- 

 plicable by the rotation which this part of the gut has undergone, 

 aided no doubt by the principle of economy of tissue which would 

 tend to bring about a shortening of the unnecessarily long pneumatic 

 duct. In some cases there still persist vestiges of the ancient cellular 

 respiratory lining of the swim-bladder (e.g. Lebiasina, Erythrinus). 



6. Finally in the Physoclistic forms — the most highly specialized 

 of all — the swim-bladder has become completely isolated from the gut, 

 its respiratory function has gone and it subserves a mainly hydro- 

 static function. 



The outline given above represents a scheme of evolution which 

 in the light of modern research has a high degree of probability. Of 

 course as in all such evolutionary speculations there exist details 

 which are still difficult to explain. While most of the facts of com- 

 parative anatomy fit in well with it, some do not — such as, for 

 example, the nerve-supply and the blood-supply of the air-bladder 

 of Amia — but it may be anticipated with considerable confidence that 

 these difficulties will be lessened or disappear with the progress of 

 research. 



1 See p. 168. This matter affords an interesting subject for further research. 



