Ill 



LUNG 



169 



the same except that line the organ is deeply bilobed: 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 



B 



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

 stage in the development of the lung. 



A, Polyptei-ns, B, Ceratodus, and C, BomUnator (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 /%7z#-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 



Th 



r/cIT 



e -ff 



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

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



.ternal pll ; 7, luiis,' ; oes, oesophagus ; part, dorsal pancreas ; p.f, pectoral limb ; Th, thyroid ; 

 '-.<. 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 Lepidosiren and Protoptcnis. 



