1V0 EMBRYOLOGY OF THE LOWER VERTEBRATES ch. 



twists upon itself, in such a way that points upon its ventral surface 

 would move towards the embryo's right side. (In other words the ■ 

 lung -rudiment rotates about its long axis in a counter-clockwise 

 direction as seen from behind, its front end remaining fixed.) The 

 two lobes are the right and the left lung-rudiment but on account of 

 the rotation just mentioned which extends through more than 180° 

 the left lobe at this stage represents what was originally the right 

 side of the rudiment. 



The two lungs of Zepidosiren or Protopterus are thus reversed in 



-int. 



Fig. 96. — Dissections of mid-gut of Lepidosiren at stages 32 (A), 35 (B), 36 (C), and 

 37 (D), showing the modelling of the intestine and also the later stages in the 

 development of the lungs. Seen from the dorsal side. 



c.c, cloacal caecum ; int, intestine ; l.l, left lung ; li, liver ; mn.d, Wolffian duct ; pan, pancreas ; 

 ph, pharynx ; r.l, right lung ; sp, spleen. 



position — the right lung of these forms being homologous with the 

 left of other Vertebrates. An important detail is that in early stages 

 the original right lung, i.e. the definitive left, is decidedly larger than 

 its fellow (Fig. 95, B). In later stages this inequality disappears, the 

 smaller lung overtaking the other in its growth (Fig. 96). 



In the case of most individuals the lungs assume their dorsal 

 position simply by growing directly tailwards, the oesophagus being 

 pushed out of the way towards the left side (Graham Kerr, 1910). 

 In certain specimens however, which doubtless in this respect retain 



