512 
J. GRAHAM KERR. 
lung on the ihght side of the body, an arrangement that must 
necessarily have been developed secondarily. 
It will be seen that neglecting the discordant evidence of 
the left vagus as we are now justified in doing, the course of 
the right vagus, right pulmonary artery, and left pulmonary 
artery agree in testifying that the lung has undergone a 
twisting on its long axis in a counter clockwise direction 
(seen from behind) during its movement to the dorsal posi- 
tion. The X-like crossing of the two pulmonary nerves 
would of course indicate that this twistina; of the lunsf 
apparatus had taken place previous to the establishment of 
the nervous “shoi't circuit” dorsal to the oesophagus. 
Having’ now shown (1) the clear probability of an ancestral 
arrangement like that of Polyp ter us becoming evolved 
into an arrangement like that of existing Lung-fishes, and 
(2) that the obstacle formed by the course of the left vagus is 
of no importance, it remains now to pass back to the evidence 
afforded by the ontogeny of Lepidosiren and Pro- 
top ter us. 
These embryological phenomena show clearly (1) that the 
lung rudiment is originally ventral in position ; (2) that 
during its development distinct twisting of the lung in a 
counter-clockwise direction takes place ; and (3) that during 
early stages of development the actual right lung, i.e. the 
lung which on Sagemehl’s hypothesis is homologous with the 
small left lung of Polyp ter us is actually much smaller than 
its fellow. 
Taking into account these various considerations we are, 
I think, irresistibly driven to the conclusion that, so far as 
regards the Dipnoi, Sagemehl’s hypothesis must be accorded 
a very high degree of probability.^ 
Actinopterygii. — The support to SagemehPs hypothesis 
which has been adduced in the foregoing paragraphs lends 
increased probability to a similar view being applicable to 
the Actinopterygians. It has been indicated how in Poly- 
* In this I agree with N EUM aye, Semon’s ‘ Zoolog. Forschungsreisen,’ 
I, p. 407. 
