STUDIES ON THE PROTOOHORDATA. 321 



first pair of nerves, as pointed oat by van Wijhe. This point is 

 of great importance, and is very strong evidence in favour of 

 the view that the hypophysis of Amphioxus (i. e. Kolliker's 

 olfactory pit) occupies a primitive position, which in the higher 

 Vertebrates has been shifted to the ventral median line by the 

 cranial flexure. 



Returning to the Protochordata, it follows from what has 

 been said above that the mouth occupies a more primitive 

 position and exhibits more primitive relations in the Ascidian 

 tadpole than it does in Amphioxus. In the larva of Amphi- 

 oxus, however, the mouth occupies an intermediate position 

 between that of the Ascidian larva and that of the adult 

 Amphioxus. Some of the stages in the migration of the mouth 

 from a dorsal to a ventral position have, in fact, been preserved 

 to us in the ontogeny of Amphioxus. The mouth of the adult 

 Amphioxus occupies the same position as that of the craniate 

 Vertebrates, but gets there by totally different means. It is 

 extremely interesting to note that there is more than one way 

 in which the primitive position of such an apparently stable 

 organ as the Vertebrate mouth can become altered, namely, 

 either by the cranial flexure or by a forward extension of the 

 notochord= 



Thus we find that in the case of the mouth of Amphioxus 

 and the higher Vertebrates we have almost identical topogra- 

 phical relations, established by widely divergent methods. A 

 similar instance is afforded by the hypophysis, which opens to 

 the exterior in the dorsal middle line in both Amphioxus and 

 Petromyzon, but primarily in the former and secondarily in 

 the latter form. 



A question may arise as to the actual way in which the 

 mouth of Amphioxus could have been originally forced aside 

 from its primitive position by the advance of the notochord. The 

 probability is that the actual oral opening was never dis- 

 placed by the notochord. But the change from a dorsal to a 

 lateral position of the mouth in the larva of Amphioxus could 

 be, and undoubtedly has been, effected by a change in the 

 order of its appearance. 



