498 
J. CiEAHAM KERR. 
in front backwards and outwards. It now forms a deep cleft 
leading right into the interior of the olfactory oi'gan along 
nearly its whole length. (The internal cavity of the olfactory 
organ is at first closed, and arises in the midst of the originally 
solid rudiment, as explained in Part 
The area containing the two olfactory clefts is now marked 
off from the rest of the under surface of the head, behind by 
a sharp fold — the first indication of the lower lip, and in 
front by the much less sharply marked rudiment of the upper 
lip. The area within these folds, and having an olfactory 
cleft upon each side, is the rudiment of that additional antero- 
lateral part of the buccal roof which becomes added on to the 
posterior and larger portion derived from the solid mass of 
yolk-cells. 
By stage XXXIV (text-fig. 6 d) the delimitation of this 
additional part of the buccal roof from the rest of the under 
surface of the head has become more sharply mai’ked, the 
upper lip being now more prominent, and the lower lip or 
lower jaw having commenced to grow forwards to form its 
floor. The olfactory cleft is more elongated. It has become 
drawn out and narrowed in its middle part to a fine slit, 
which connects the dilated anterior and posterior ends — the 
rudiments of the anterior and posterior nares. Of these the 
posterior naris is now hidden in a ventral view of the head, 
owing to the forward growth of the lower jaw. 
In stage XXXV it is necessary to cut away part of the 
lower jaw to see the olfactory clefts. It is seen (text-fig. 6 e) 
that the lips of the cleft are now in close apposition except 
at their ends, and between this stage and stage XXXVI the 
lips undergo complete fusion, so that the anterior and poste- 
rior nares are now distinct openings (text-fig. 6 p). 
Keibel has remarked:^ “The so-called upper lip of the 
Dipnoi lies morphologically further outwards than does the 
mouth margin in any other Vertebrates.” This character is 
specially expressed in the fact that the margin of the mouth 
' ‘ Quarterly Journal of Microscopical Science,’ vol. 46, p. 438. 
2 ‘ Anat. Anz.,’ vol. viii, p. 487. 
