LARVAL DEVELOPMENT. \6l 



Larval Asymmetiy ttot Adaptive and not Advantageous ; 

 Forzvard Extension of NotocJiord Adaptive and Advan- 

 tageous. 



The conclusion to be drawn from the above considera- 

 tions is that the remarkable asymmetry of the larva of 

 Amphioxus, in respect of the pharynx and the parts con- 

 nected with it, is of no specific advantage whatever to the 

 larva, but is merely a stage, which has been preserved in 

 the ontogeny, of a topographical readjustment of parts 

 necessitated by the removal of the mouth from its primi- 

 tive mid-dorsal position in consequence of the secondary 

 forward extension of the notochord, which has thus caused 

 a virtual semi-rotation of the pharyngeal region of the 

 body. On the other hand, the forward extension of the 

 notochord is a distinct advantage in later life, since, by 

 giving resistancy to the snout, it enables the animal to 

 burrow its way into the sand with such astonishing facility, 

 while the fact that it grows to the front end of the body at 

 a very early stage in the embryonic development, long 

 before it comes to be put to this definite use, must be 

 regarded as an instance of /'rr6-i7(:?(?«i' development of which 

 there are numerous and otherwise inexplicable examples 

 in the field of comparative embryology. 



The larval asymmetry of Amphioxus is therefore a purely 

 secondary or cenogenetic feature, and has no directly ances- 

 tral or palingenetic significance, although, as we have shown 

 above, it serves indirectly as a clue to what the ancestral 

 condition was. At the same time it is a primary feature 

 in the actual ontogeny ; that is to say, the asymmetrical 

 structures (mouth and gill-slits) arise in situ, and are not 

 removed in the individual development from a primary 



