ALIMENTARY CANAL IN LEPIDOSIEEN AND PROTOPTEEUS. 507 
occurrence of these two torsional processes in opposite direc- 
tions introduce, as will readily be understood, very puzzling 
and deceptive appearances into the sections of larvae of such 
stages of development. 
Topographical Relations of Lung to Coelom. — In 
its earliest stages the endodermal lung rudiment is naturally 
enclosed within the mesenchymatous tissue of the splanchno- 
pleure. In a Protopterus of stage XXXIV the root of the 
lung retains these relations. If, however, sections farther back 
towards the hind end of the lung rudiment are examined it is 
found that the oesophagus has, in this region, bent away 
towards the left side, and has become freed from the dorsal 
mesentery, which passes direct from the under surface of the 
dorsal aorta to the upper surface of the liver. The lungs 
grow directly backwards in the substance of this dorsal 
mesentery. (It may be mentioned incidentally that in Poly- 
pterus the hind portion of the large right lung retains 
throughout life this relatively primitive position in the sub- 
stance of the mesentery.) 
The dorsal mesentery undergoes a remarkable process of 
thickening from side to side, forming a broad mass of spongy 
connective-tissue bounded superficially by ccelomic epithelium. 
It is specially broad dorsally, and it is in this specially bi'oad 
dorsal region that the lungs are situated, so that as the 
broadening of the base of attachment of mesentei’y to dorsal 
body-wall goes on the originally dorsal part of the mesentery 
(containing the lungs) becomes gradually completely merged 
in the roof of the splanchnocoele. The lungs thus come to be 
situated outside of and completely dorsal to the splanchnocoele. 
General Discussion of the Morphology of the 
Lungs in Lepidosiren and Protopterus. — I feel com- 
pelled to accept the general homology of the organs known 
in various subdivisions of the Vertebrata under the name of 
lung and swim-bladder or air-bladder. The early stages of 
obviously suggested that the spiral coiling may once have extended 
forwards into the region of the oesophagus, and that the primary torsion 
has persisted on the straightening out of the spiral coils. 
