444 EMBEYOLOGY OF THE LOWER VERTEBRATES oh. 



Anguis) in those various groups of Lizards which have developed 

 an elongated snake-like form. So also in Snakes. 



In cases where the limbs are completely gone in the adult it may 

 be possible to observe them during early stages of development. 

 The young Symbranchus (Fig. 203) has for a time huge pectoral fins 

 which it uses as organs of respiration (Taylor, 1914). In Gymno- 

 phiona and Blind worms {Anguis) 1 minute limb rudiments have also 

 been observed in the embryo. In other cases no trace of the missing 

 limbs has been found during early development. In view of this 

 general tendency of the limbs to disappear in Vertebrates which 

 have assumed an eel-like or snake-like form of body it is well, as 



Fig. 203. — Symbranchus marmoratus. Larvae showing pectoral fins. 

 (After Taylor, 1914. ) 



o.o, opercular opening; s.i.v, subintestinal vein ; y, yolk. 



already indicated, not to assume a dogmatic attitude in regard to 

 such Vertebrates as Lampreys or Hag-fishes. The possibility is not 

 excluded that even these Cyclostomes are descended from ancestors 

 in which limbs were present. 



The interesting question now emerges — How did the limbs of the 

 Vertebrate originate in evolution ? Few morphological speculations 

 have excited more interest and more controversy than this. Two 

 main hypotheses have been propounded and each has found supporters 

 amongst the most eminent morphologists. Although in the opinion 

 of the present writer it is no longer necessary to fall back upon 

 either of these views, a simpler possibility having presented itself, a 



I, 1 The elementary student may be warned not to mistake the rudiments of the 

 paired penes in Snake embryos for limbs ! 



