832 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 



does not appear to undergo any further development, and, so far 

 as we can make out, disappears shortly after the embryo is 

 hatched, without acquiring an opening to the exterior. 



It is important to notice that this cleft, which in the cartila- 

 ginous 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- 

 glandular anterior region forming the oesophagus. The lumen 

 of this part appears to be all but obliterated in the stages im- 

 mediately before hatching, giving rise for a short period to a 

 solid oesophagus like that of Elasmobranchii and Teleostei 1 . 



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 40, 

 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 40, fig. 65, a.b.}. In 

 the larva of a month and a half old (26 millims.) it has still a 

 perfectly simple form, and is without traces of its adult lung-like 

 structure ; but in the larva of 1 1 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 continuous be- 



1 Vide Coinp. Embryo!., Vol. II., pp. 50 63 [the original edition]. 



