320 ARTHUR WILLEY. 



the secondary forward extension of the notochord from the 

 hypophysis. In Petromyzon the whole process can be observed, 

 while in Amphioxus only part of it, as the notochord grows 

 forward at a very early stage before the formation of the mouth. 



I take it for granted, therefore, that the mouth of Amphi- 

 oxus was primitively dorsal, and the prime reason of its being 

 dorsal was, not the presence of a prseoral lobe or anterior 

 body- cavity, but the fact that in the common ancestor of the 

 Urochorda and Cephalochorda the mouth stood in intimate 

 relation with the neuroporus, probably through the inter- 

 mediation of a ciliated funnel or hypophysis. This conclusion 

 would suit very well with the views oi' Sedgwick (19) and 

 van Wijhe (21) as to the primitive respiratory function of the 

 neural canal, water entering it by the neuroporus which opened 

 into the mouth, and leaving it by the neurenteric canal. 



As to the position of the mouth in the higher Vertebrates, 

 it is obvious, supposing the above considerations to be correct, 

 that it has, so to speak, been pushed round to its present ven- 

 tral or subterminal position by the cranial flexure. This was 

 first suggested in part by Sedgwick in his well-known paper on 

 the " Origin of Metameric Segmentation,'^ although the mouth 

 of Amphioxus, whose final ventral position is due to an entirely 

 diff'erent set of causes, was left out of consideration. He says 

 (18, p. 77), "With a slight change in the shape of the ante- 

 rior end of the body of the Ascidian larva in Kowalevsky's 

 figure, the mouth would be removed from what we call the 

 dorsal (neural) to what we call the ventral (abneural) surface. 

 This would involve a flexure of the anterior end of the neural 

 canal, and, I think, gives a clue to the phylogenetic meaning 

 of the cranial flexure." 



As for the higher Vertebrates, my friend Mr. H. B. Pollard 

 had the kindness to show me in Naples some of his prepara- 

 tions of Teleostean embryos, in which it could readily be seen 

 that the hypophysis was morphologically dorsal with reference 

 to the nervous system, its actual ventral position being due to 

 its having been carried round the front of the head by the 

 cranial flexure, just as the optic nerves are morphologically the 



