472 



G. RYLANDS DE BEER 



Reighard, and Pliclps (16) havo described tho sucker as arising 

 from muscle anterior to the premandibular somite. Goodrich 

 (7) has produced good evidence to show that the anterior head 

 diverticula of Amphioxus are homologous with the premandi- 

 bular somites of Craniates, and it is more reasonable to agree 

 with Dohrn (3) that mo segmental significance must be attached 

 to Piatt's anterior head cavities. Then, supposing that the 



Text-fig. 13. 

 Tr N4 F Ns Ne N/GI V Sg^ §7 



^ 



imm. 



Embryo 6 mm. long showing the relations of the neuroiueres to the 

 remaining segmented structures. 



subdivision of the third neuromere is not secondary but 

 primary and retarded (which it might be, for tlie third neuro- 

 mere is just about twice as long as the following ones), it would 

 be necessary to postulate yet another gill-slit lost, to correspond 

 to the extra neuromere. But perhaps the greatest objection 

 to the segmental value of neuromeres lies in the fact that they 

 are altogether absent in Amphioxus, scarcely developed in 

 Petromyzon, irregular and asymmetrical in Bdellostoma, and 

 that they are best developed in the higher craniates, birds, 

 and mammals (Neal, 13). This strongly discountenances their 

 palingenetic value and suggests that they aie neomorphs. 

 And so I cannot believe that the evidence from neuromeres 

 favours the assumption of a lost gill-slit. 



