166, EMBEYOLOGY OF THE LOWEE VEETEBEATES oh. 



growths arise from the walls of the air-sacs which burrow through 

 the neighbouring tissue, even through bone, taking the place of the 

 marrow and rendering the bones pneumatic. Such outgrowths may 

 extend even into the terminal phalanges of the digits. They may 

 also extend in amongst the connective tissue of the skin or between 

 the muscles. 1 



THE LUNG IN FISHES 



In the typical fishes or Teleostei, which of all Vertebrates are 

 the most highly specialized in adaptation to a purely swimming 

 habit, one of the most characteristic organs is the swim-bladder or 

 air-bladder. In its most highly developed form (in the physoclistic 

 Teleosts) this consists of a closed sac, lying above — dorsal to — the 

 splanchnocoele and filled with gas containing a large proportion 

 of oxygen. Special developments of the lining epithelium provide 

 a mechanism whereby the amount of gas in the organ can be 

 increased by a process of secretion or diminished by a process of 

 absorption. 



This mechanism, which is under the control of the nervous 

 system, has for its main function the keeping the body of the fish 

 at the same constant specific gravity as the water in which it is 

 swimming — counteracting changes in its specific gravity which 

 would otherwise result from variations of pressure due to change of 

 depth, or from variations in volume of gas produced by fermentative 

 processes in the alimentary canal. The air-bladder with its com- 

 pensating mechanism keeps the fish precisely at the specific gravity 

 of the surrounding medium so as to obviate the expenditure of 

 muscular effort in order to keep at one depth such as is necessary in 

 the case of a shark or other fish unprovided with an air-bladder. 



The air-bladder arises in development as an outgrowth of the 

 wall of the alimentary canal behind the region of the gill-clefts. 

 This burrows its way backwards dorsal to the splanchnocoele and 

 eventually attains to a large size. Its tubular connexion with the 

 alimentary canal (pneumatic duct) becomes constricted across and 

 severed so that the organ is completely isolated from the alimentary 

 canal. In a good many cases however — namely, in the physosto- 

 matous Teleosts — the duct persists and remains patent throughout 

 life. 



In many fishes the dorsal wall of the air-bladder bulges out in a 

 headward direction (Fig. 93) forming a diverticulum which may 

 reach a great size so that in the adult the organ has the appearance 

 of being composed of two segments marked off from one another by a 

 constriction (Fig. 93, B), the pneumatic duct communicating with 

 the hinder one of the two. The constriction may become accentuated 



1 Since the above account was written a full and well-illustrated description of the 

 development of the Fowl's lung has been published by Locy and Larsell {Amer. Joum. 

 Anat., xix and xx, 1916). These authors' results amplify and in the main confirm 

 those of Juillet. 



