508 
J. GRAHAM KERR. 
development of this organ in the ordiuai*y lung-breathing 
animals, in Lung fishes, and in the most archaic existing 
Teleostome — Polyp ter us — are identical in their main 
features, the differences which are so conspicuous in the 
fully-developed organ of the adult being of a purely secondaiy 
character. For convenience I will use the term lung to 
express the organ alluded to in whatever form it occurs. 
It must further, I think, be conceded that the original 
position of the lung was ventral. This can hardly be denied 
in view of the fact that in both Crossop terygians and Lung 
fishes — both of them archaic groups in which the lung plays 
an important hydrostatic function and has, in correlation 
with this, assumed in the adult, partially or completely, a 
position dorsal to the other viscera of the splanchnocoele — the 
whole lung rudiment is in early stages of development, as is 
the glottis throughout life, ventral in position. 
But, admitting these two points, there at once ai-ises the 
further question as to the exact method by which the dorsal 
position of the lung, e. g. of a Teleostean fish, has come about 
in phylogeny, and this carries with it other questions as to 
the precise hoinology of the parts of the dorsal lung- of such 
forms with those of the ventral lung. 
It was Sagemehl ^ who first propounded the view which, in 
its main features, finds a striking corroboration in the facts 
of ontogeny and adult anatomy of the Dipnoi. Taking the 
bilobed mid-ventral condition of the lung as a relatively 
primitive one, Sagemehl points to the condition in Poly- 
pterus in which the left lobe or left lung has become 
greatly reduced, as is so frequently the case in lung-breathing 
Vertebrates with elongated bodies. Were this lopsided con- 
dition of the lung can-ied further, and the left lobe reduced 
to relatively insignificant dimensions, it is obvious that there 
would no longer be any insuperable difficulty in imagining a 
dorsalward shifting of the remaining right lung round the 
right side of the oesophagus, so that eventually a dorsal 
position might be attained, as in e. g. Ceratodus. Sagemehl 
* ‘ Morpliol. Jalirh.,’ x, 1885, p. 108. 
