STRUCTURE AND DEVELOPMENT OE LEPIDOSTEUS. 
427 
It is important to notice that this cleft, which in the cartilaginous Ganoids and 
Polypterus remains permanently open as the spiracle, is rudimentary even in the 
embryo of Lepidosteus. 
The second pouch is the hyobranchial pouch: its outer end meets the epiblast before 
the larva is hatched, and a perforation is effected at the junction of the two layers, 
converting the pouch into a visceral cleft. 
Behind the hyobranchial pouch there are four branchial pouches, which become 
perforated and converted into branchial clefts shortly after hatching. 
The region of the oesophagus following the pharynx is not separated from the 
stomach, unless a glandular posterior region (vide description of adult) be regarded as 
the stomach, a non-glanular anterior region forming the oesophagus. The lumen of 
this part appears to be aJl but obliterated in the stages immediately before hatching, 
giving rise for a short period to a solid oesophagus like that of Elasmobranchii and 
Teleostei.* 
From the anterior part of the region immediately behind the pharynx the air- 
bladder arises as a dorsal unpaired diverticulum. From the very first it has an 
elongated slit-like mouth (Plate 27, fig. 64, a.b'.), and is placed in the mesenteric 
attachment of the part of the throat from which it springs. 
We have first noticed it in the stages immediately after hatching. At first very 
short and narrow, it grows in succeeding stages longer and wider, making its way 
backwards in the mesentery of the alimentary tract (Plate 27, fig. 65, a.b.). In the 
larva of a month and a half old (26 millims.) it has still a perfectly single form, and 
is without traces of its adult lung-like structure ; but in the larva of 11 centims. it 
has the typical adult structure. 
The stomach is at first quite straight, but shortly after the larva is hatched its 
posterior end becomes bent ventralwards and forwards, so that the flexure of its 
posterior end (present in the adult) is very early established. The stomach is con¬ 
tinuous behind with the duodenum, the commencement of which is indicated by the 
opening of the bile duct. 
The liver is the first-formed alimentary gland, and is already a compact body before 
the larva is hatched. We have nothing to say with reference to its development, 
except that it exhibits the same simple structure in the embryo that it does in 
the adult. 
A more interesting glandular body is the pancreas. It has already been stated 
that in the adult we have recognised a small body which we believe to be the pancreas, 
but that we were unable to study its histological characters. 
In the embryo there is a well-developed pancreas which arises in the same position 
and the same manner as in those Yertebrata in which the pancreas is an important 
gland in the adult. 
We have first noticed the pancreas in a stage shortly after hatching (Plate 27, 
* Vide ‘ Comp. Embryo].,’ vol. ii., pp. 50-63. 
