The absence of a pneumatic duct in the Physoclisti would appear to be due to a post-larval atrophy.
In Lepidosteus the air-bladder appears to arise, as in the Teleostei, as an invagination of the dorsal wall of the œsophagus.
In advanced embryos of Galeus, Mustelus and Acanthias, Miklucho-Maclay detected a small diverticulum opening on the dorsal side of the œsophagus, which he regards as a rudiment of a swimming bladder. This interpretation must however be regarded as somewhat doubtful.
The lungs. The lungs originate in a nearly identical way in all the Vertebrate forms in which their development has been observed. They are essentially buds or processes of the ventral wall of the primitive œsophagus.
At a point immediately behind the region of the visceral clefts the cavity of the alimentary canal becomes compressed laterally, and at the same time constricted in the middle, so that its transverse section ([fig. 418] 1) is somewhat hourglass-shaped, and shews an upper or dorsal chamber d, joining on to a lower or ventral chamber l by a short narrow neck.
The hinder end of the lower tube enlarges ([fig. 418] 2), and then becomes partially divided into two lobes ([fig. 418] 3). All these parts at first freely communicate, but the two lobes, partly by their own growth, and partly by a process of constriction, soon become isolated posteriorly; while in front they open into the lower chamber of the œsophagus ([fig. 422]).
Fig. 418. Four diagrams illustrating the formation of the Lungs. (After Götte.)
a. mesoblast; b. hypoblast; d. cavity of digestive canal; l. cavity of the pulmonary diverticulum.
In (1) the digestive canal has commenced to be constricted into an upper and lower canal; the former the true alimentary canal, the latter the pulmonary tube; the two tubes communicate with each other in the centre.
In (2) the lower (pulmonary) tube has become expanded.
In (3) the expanded portion of the tube has become constricted into two tubes, still communicating with each other and with the digestive canal.
In (4) these are completely separated from each other and from the digestive canal, and the mesoblast has also begun to exhibit externally changes corresponding to the internal changes which have been going on.
By a continuation forwards of the process of constriction the lower chamber of the œsophagus, carrying with it the two lobes above mentioned, becomes gradually transformed into an independent tube, opening in front by a narrow slit-like aperture into the œsophagus. The single tube in front is the rudiment of the trachea and larynx, while the two diverticula behind become ([fig. 419], lg) the bronchial tubes and lungs.