LUNG-FISH 



391 



two pairs of these external gills have survived in the modern vertebrates 

 owing to their having developed supporting and locomotor functions in 

 place of their original respiratory one and that these have evolved into 

 the two pairs of limbs. This interesting problem is gone into in detail 



FiG.;i7i. 



Larval stages of Lepidosiren. c.o, Cement-organ ; e.g, external gill ; M, mouth ; p.f, pectoral fin ; 

 pl.fj pelvic fin ; sp.v, spiral valve of intestine. (A-D X 3 ; E X 2-5). 



in my Embryology and all that need be said here is that in my opinion 

 the balance of probability is on the side of this " external gill " hypothesis 

 being actually the true theory of the evolutionary origin of the vertebrate 

 limbs. 



Except in the case of Ceratodus the dipnoan larva also possesses a char- 

 acteristic cement-organ (Fig. 171, D, c.o) — a large cushion-like structure 



