Neal, Problem of the Vetteh^ate Head. 1 5 7 



dorsal nerves cannot be of the same kind ; and other less im- 

 portant differences by no means outweigh the evidence of simi- 

 larity of head and trunk segments. 



As a mater of fact some of the differences alleged above 

 do not actually exist. Many, is is noted, apply to the nerves, 

 and these have seemed so great that even Gegenbaur, the early 

 champion of the present morphological conception of the ver- 

 tebrate head states i^'^'J^ that he is no longer able to consider 

 cephalic and spinal nerves as homodynamous. With our pres- 

 ent knowledge, however, that in Amphioxus two kinds of 

 nerves, viz. dorsal mixed nerves whose motor fibers innervate 

 splanchnic musculature, and ventral motor nerves which inner- 

 vate somatic musculature, are found in each segment of the 

 body except the first ; that in Craniota both of these kinds of 

 nerves appear in the head as well as in the trunk ; that a pair is 

 to be found in each trunk metamere (in Petromyzon unconnect- 

 ed as in Amphioxus), and in some head metameres, I am una- 

 ble to regard the actual differences between cephalic and spinal 

 nerves as fundamental in character.^ 



The differences which appear are, in my judgment, to be 

 expected in the case of the nervous organs in such highly differ- 

 entiated structures as head and trunk. Furthermore, the fact that 

 the bounds of head and trunk in the vertebrate series are not 

 definitely fixed ; that they are variable ; that there is an un- 

 broken continuity throughout head and trunk of such essential 

 components of metameres as neuromeres, nerves, somites, vis- 

 ceral arches, visceral clefts, and aortic arches, is evidence suffi- 

 cient to warrant the general belief in the serial homology of the 

 segments in these two regions. So far as I can see, no objec- 

 tions to this view apply to the pre-otic region which are not equal- 

 ly applicable to the post-otic region. If the segments in the 

 one region are serially homologous with trunk metameres, those 

 in the other region are also. I shall be obliged to refer the 



^ The evidence both histological (Lenhossek, Kolliker, Ramon y Cajal) and 

 physiological (Steinach and Wisner) given in the last decade, seems to establish 

 conclusively the fact (rendered a /rtVrz probable by the evidence from Amphi- 

 oxus) that spinal dorsal nerves are like cephalic dorsal nerves mixed in function. 



