126 J. B. JOHNSTON. 



(e) The viscero-motor cells are situated as in vertebrates dor- 

 sal to the somatic motor cells, lateral to the ventral part of the 

 canal. 



(/) The nerve cells retain the position and characters which 

 are typical in the embryos of vertebrates and which are seen in 

 certain parts of the brain of many fishes. 



(g} The ventral roots arise separately and remain independ- 

 ent. They are true somatic motor nerves. 



We have here nothing else than an essentially vertebrate type of 

 nervous system. At the same time there are good indications of 

 a truly primitive, unspecialized condition. In addition to the 

 facts given under a, c and/, there may be mentioned : 



(Ji) The total absence of certain specialized structures wnich 

 characterize all vertebrates, namely, hair cells responding to 

 vibrations in fluid (neuromasts or acustico-lateral organs), and 

 retinal visual organs. (The morphology of olfactory and gus- 

 tatory sense organs is yet in an uncertain state.) 



(/) The presence of simple light-percipient organs within the 

 central nervous system, which have apparently been retained 

 from the worm-like ancestors of Amphioxus. 



(_/) The very slight development of the brain. 



These facts speak eloquently against the supposition that 

 Amphioxus is the result of a process of degeneration from any 

 form which had reached a higher degree of specialization, such 

 as selachians or other fishes. Similar facts have been brought 

 forward (6, p. 73) to show the primitive character of the cyclo- 

 stome brain. We cannot suppose that specialized structures in 

 the nervous system once possessed by the ancestors of Amplii- 

 oxus have been nicely pruned back and reduced in the process 

 of degeneration ; nor can we believe that true nephridia and 

 helminthine eye-spots should reappear in a degenerated species 

 whose ancestors in the course of their evolution had lost these 

 very organs. The straightforward interpretation of the nervous 

 system supports the view that Ampliioxns and Cyclostomes are 

 the lower branches of the vertebrate phylum. * 



1 The Worcester's fluid spoken of in the text is as follows : 



40 per cent, formalin IO parts, 



Distilled water 90 " 



Saturate this with sublimate ; 



Add glacial acetic to make IO per cent. 



