Ill 



LUNG 



169 



the same except that here the organ is deeply bibbed : a right and a 

 left lung or air-bladder occupying the place of the single organ of 

 Ceratodus. 



The meaning of the ventral position of the glottis in these Lung- 

 fishes, and, in fact, the morphological nature of the whole organ, is 



ph. 



ph. 



/. 



1 



Fig. 94. — Transverse sections through the endoderm of the pharynx showing an early 

 stage in the development of the lung. 



A, Polyplerus, B, Ceratodus, and C, Bombinator (C after Goette, 1875). I, lung-rudiment ; 



ph, pharynx. 



demonstrated by the examination of early stages in development. 

 In these the organ is found to be a perfectly typical few^-rudiment 

 (Fig. 94, B) — a mid-ventral projection from the pharyngeal floor of 

 precisely the same kind as that found in tetrapodous vertebrates (C). 1 



Fig. 95. — Views showing early stages of the lung-rudiment of Protopterus as seen from 

 the ventral side (stages xxxii, xxxiv, xxxv). 



e.g, external gill ; I, lung ; oes, oesophagus ; pan, dorsal pancreas ; p.f, pectoral limb ; Th, thyroid ; 

 v.c, visceral cleft rudiment. (Cut surfaces are indicated by uniform light tone.) 



Subsequent stages are illustrated by Figs. 95 and 96. The lung 

 rudiment at first a rounded knob (Fig. 95, A) grows backwards and 

 soon becomes bilobed (B). The figure does not bring out one im- 

 portant fact namely that the lung-rudiment as it grows backwards 



1 The projection is at first solid in the case of Zepidosiren and Protopterus. 



