﻿170 Proceedings of the Royal Physical Society. 



XV. Note on Swim-hladder and Lungs. 

 By Prof. J. Geaham Kerr. 



(Read 23rd March 1908.) 



The general homology of swim-bladder and lungs is now 

 accepted by most morphologists as being well established. 

 There still remains, however, a good deal of divergence of 

 opinion as to the precise way in which one has been derived 

 from the other. I have, for some time past, been inclining 

 towards the view that the hypothesis enunciated by Sagemehl 

 is the correct one. According to this view, the condition in 

 which there exists a pair of lungs with a mid- ventral glottis 

 is the primitive one. Since Sagemehl's time this has 

 received support from the facts of embryology of Crossop- 

 terygians and Dipnoans. According to Sagemehl, the 

 increasing predominance of the hydrostatic function of the 

 lungs in fish-like forms was accompanied by the gradual 

 disappearance of one lung and the enlargement of the 

 remaining one, which was now able to pass up to a mid-dorsal 

 position round one side of the oesophagus. Sagemehl points 

 to Polypterus as showing an early stage in this process, 

 the left lung being here reduced and the right greatly 

 enlarged. It is to be noted that in Polypterus, although 

 the lung apparatus shows in detail a strong morphological 

 asymmetry, yet the arrangement of the parts as a whole 

 is practically symmetrical — the large right lung being 

 quite symmetrical about the mesial plane except at 

 its front end, where it bends away to the right and is 

 balanced by the small left lung. This symmetry of the 

 lung apparatus as a whole, in spite of the asymmetry of its 

 constituent parts, is an expression of the important hydro- 

 static function of the apparatus. 



A consideration of the morphological features of the lung 

 or swim-bladder of the Dipnoans shows at once that 

 Sagemehl's hypothesis fits in well with certain of their 

 characteristic features. Descent of the lung-fishes from an 

 ancestor in which the lung apparatus was in a similar 

 condition to that in which it is in the living Polypterus, 



