Johnston, Morphology of the Head. 215 



of N.VII which is unknown. (But see 73). The position of 

 N. VI and the absence of a general cutaneous component have 

 not yet been accounted for. N. VI innervates the muscles de- 

 rived from somite 3 and its nucleus of origin and root are found 

 at approximately the same level with N. VII in all vertebrates 

 except Petromyzon. Whether its usual position is to be account- 

 ed for by supposing that it has shifted back at the same time 

 with the other nerves, or by supposing that it represents the 

 ventral nerves of two segments requires further investigation. 

 I am inclined to think that Neal's reasoning is right in this 

 matter and that the latter is the more probable interpretation. 

 However, no support for this was found by Koltzoff in the de- 

 velopment of Petromyzon where we should have most expected 

 it. The writer is convinced that the adult relations of N. VI in 

 Petromyzon are not correctly described and he hopes soon to 

 re-investigate them. 



For the explanation of the absence of a general cutaneous 

 component from this segment, also, we must await further in- 

 vestigation. Such a root should be present for the innervation 

 of the skin around and above the hyomandibular cleft in cyclo- 

 stomes and selachians, and in operculated forms for the inner- 

 vation of the operculum. The hypothesis may be suggested 

 that in the process of shifting above described the general cu- 

 taneous fibers which originally accompanied N.VII remained in 

 neuromere vii and were absorbed by N. V, so that the general 

 cutaneous fibers of three segments are concentrated at the pro- 

 fundus-trigeminus root. The fact that a part of the operculum 

 in teleosts (46) is innervated by the trigeminus is consistent with 

 this hypothesis. If the corresponding area in cyclostomes and 

 selachians should have the same innervation, it would give 

 strong support to the suggestion just made. 



10. Segment 7. The aciistico-lateral system. 



In the primitive condition this segment was probably com- 

 posed of the following elements: neuromere vii, somite 3, hyoid 

 arch, somatic motor N. VI, splanchnic motor N. VII, general 

 cutaneous component ( ?), and splanchnic sensory N. VII, with the 



