ORIGIN AND ST RUCTURE OF THE HEAD 93' 



oral fusion outlined in fig. 1" (reproduced here in fig. 19). 

 Miss Platt points to the circumstance that in Necturus 

 the rudimentary hyomandibular pockets and the hyobranchial 

 clefts also fuse with one another ventrally. The definitive 

 shape of the mouth is established by the elongation of the 

 lateral extension of the oral fusion, while the longitudinal 

 part of the latter becomes obliterated. 



The relation of the trigeminus to the mouth closely corres- 

 ponds to that of the facialis to the spiraculum and that 

 of the glossopharyngeusand the vagus 

 to the following gill-slits. If indeed 

 the mandibular segment is to be consi- 

 dered as the first segment of the soma, 

 the above cited objection of ZlEGLER 

 against the derivation of the mouth 

 from two gill-clefts wholly looses it 

 validity. It is especially the conclu- 

 sions we shall reach in regard to 

 Fig. 19. Formation of Amphioxus (cf. anon) which plead for 

 the mouth in Necturus the assumption that the gill-slits are 

 ^^^YoQ^'ss Platt, indeed older than the mouth and that 

 i«y5, p. 559. ^j^g jg^^gj. jg ^^ ^^ derived from them. 



The nerve belonging to the first pair of gill-slits, repre- 

 sented by the mouth, would be then the trigeminus, which thus 

 ought to be considered as a single segmental nerve and not as 

 a double one, which would imply the falling out or failing of 

 a pair of prae-oral gill-slits, of which in Craniates there is 

 no indication. The scheme we arrive at in this way is still 

 simpler than that of ZlEGLER and closely approaches that of 

 GegENBAUR after he had discarded his view that the labial 

 cartilages represent two praeoral visceral archs (1887, p. 79). 

 The visceral archs appear now indeed to correspond to the 

 segments incorporated into the head, the mandibular arch 

 representing the first or trigeminus-segment, the hyoid arch 

 the second or facialis-acusticus segment, the first branchial 

 arch the third or glossopharyngeus segment, etc. With 

 HATSCHEK (1892, p. 159, 160) we reach the conclusion 

 that the palaeocranium of Ammocoetes, into which the 

 glossopharyngeus and the vagus have not yet been incor- 

 porated, comprises only two segments, viz: the trigeminus- 

 and the facialis-segment, besides the prostomium, which by 

 HATSCHEK is not distinguished from the first segment, 

 his "acromerite". 



