in LUNG 161 



more probable that the evolution of these organs has been in the 

 opposite direction. 



On this latter hypothesis the external gills would be regarded 

 as the primitive respiratory organs, inherited probably from pre- 

 vertebrate ancestral forms. The evolution of clefts between their 

 bases would be explicable as an arrangement for pumping water 

 over the surface of the external gills, while it could be readily 

 understood that the respiratory tissue would then tend to spread 

 .inwards along the lining of the clefts, where it would be both 

 advantageously situated for carrying out its breathing function and, 

 at the same time, protected from the dangers to which external gills 

 are exposed. The development of respiratory lamellae to increase 

 the area of this respiratory tissue on the wall of the cleft would be a 

 further and natural development. 



The chief difficulty in the way of accepting this as a working 

 hypothesis lies in the existence of animals admittedly near the base 

 of the Vertebrate scale — such as Amphioxus and the Cyclosto- 

 mata — in which there are no external gills and no vascular 

 yolk-sac to account for their disappearance. This difficulty is 

 undoubtedly a serious one but on the whole the present writer is 

 inclined to think the difficulty is not so great as to justify the 

 immediate rejection of the hypothesis : it becomes less formidable 

 when it is borne in mind that the forms mentioned although 

 evidently archaic in some of their characteristics bear in others 

 equally convincing evidence of high specialization. 



Lung. — In all the groups of Gnathostomata excepting the Elasmo- 

 braneh fishes the pharyngeal wall develops a great outgrowth 

 which, as will become apparent later, is to be looked upon as' homo- 

 logous throughout the series and as primarily respiratory in its 

 function — the lung. The lung appears in its most familiar and 

 typical form in the tetrapod Vertebrates and its development in 

 these will accordingly be considered first. 



Here in an early stage of its development the lung is in the 

 form of a pocket of the pharyngeal floor projecting downwards in 

 the mid - ventral line. This pocket commonly makes its first 

 appearance as a longitudinal groove or gutter in the floor of the 

 pharynx at about the level of the last visceral cleft. The groove 

 becomes constricted off from behind forwards, so as to form a 

 blindly ending pocket communicating in front with the pharyngeal 

 cavity by a narrow opening — the glottis— and extending back 

 immediately ventral to the pharynx. The blind end of the pocket 

 grows actively tailwards and becomes deeply bilobed — the two 

 lobes becoming respectively the right and left lung, while the 

 unpaired portion connecting them with the glottis becomes the 

 trachea or pneumatic duct. 



While the lung passes in its early history through stages corre- 

 sponding on the whole with those described there are differences in 

 detail in different groups- -the most conspicuous of these variations 



vol. II M 



