568 



The most interesting change however caused by the downward 

 and backward growth of the brain, affects the visceral mesoderna. 

 The anterior arches have been bent downwards, and all have practi- 

 cally disappeared in the Craniates except the last of the series which 

 forms the mandibular arch. 



Parachordal 

 Auditory Capsule 



Lateral Sphenoid 



- - Epiphysis 



3Iandibiihir 



Trabe cilia 



Fig. 6. Acanthias. Founded on a graphic reconsti'uction by Sewektzoff. The 

 cartilages are shown in dark shading. The nerves and the branchial clefts and arches 

 are indicated by numbers. 



Branchiomeres. 



It is necessary before considering the gill arches further to recall 

 the above results with regard to the ganglia. The attempt was made 

 to show that in relation to the brain segments there were, from the 

 3rd prosomere to the 3rd rhombomere, 6 ganglia, which presumably 

 were associated, like the other post-stomial ganglia, with visceral clefts 

 and arches. If the mouth were originally confined to the first of the 

 segments then the mesencephalic flexure has brought about the loss of 

 all the arches until we come to that one associated with the mandi- 

 bular V. Reverting to the nomenclature given above when the nerves 

 were considered, the primitive mouth was more than likely innervated 

 by the thalamic, and as soon as the mesencephalic flexure established 

 its Craniate position, the mesencephalic, trochlear, and V^ and V^ 

 clefts and arches were absorbed or modified. The labial cartilages 

 remain in the lower Craniates to remind us of the gills and gill arches 

 involved in the mouth area, and also to suggest that the non-sensorial 

 portion of the nose may owe somewhat to gill cleft also. 



