No. 470] 



UNITY OF GNATHOSTOME TYPE 



87 



"The large collections of ganglion cells just posterior to the 

 thalamocoele are homologous with the medullary nuclei of other 

 vertebrates, since their connections show them to be centers for 

 the control of the branchial apparatus, and the sensory and motor 

 structures lying in the territory of the gill basket, e. g., centers of 

 respiration, deglutition, etc." 



"The ontogenetic changes of the neural axis in other vertebrates 

 carry the brain through the condition which in amphioxus remains 

 permanent as the adult brain." 



As regards the eye, I announced in 1891 that the eye-spot of 

 amphioxus — that is to say, the unpaired but bilaterally symmet- 

 rical patch of epithelial cells lying in the lamina terminalis of the 

 amphioxus brain — is the forerunner of the vertebrate eye, and 

 that, as regards its physiology, it was not a visual organ nor an 

 organ of sight, but an organ for the perception of the variations 

 in the intensity of light. 



This pigmented patch of epithelium occupies the same ])f)sition 

 in the adult amphioxus that the unpaired but bilaterally symmet- 

 rical patch of pigmented cells in the embryo sturgeon, as described 

 by Kupffer, and in the embryo of Galeus, as observed by me, does 

 with reference to the lamina terminalis of the brain of these forms. 



In both the latter cases the pigmented patch is converted into 



ess of evagination to the two optic vesicles. 



Amphioxus, therefore, presents us with an adult condition 

 which is represented in the higher vertel)rate form l)y the simple 

 condition of the brain wall in the earlier stages of the development 

 of the nervous system. 



For a fuller discussion of the anatomical conditions present in 

 the adult amphioxus see my paper (loc. cit., pages 238 to 234). 



It is clear from this description of the lamina terminalis in the 

 embryos of the sturgeon and of the dog-fish that the early stage 



with the eye oi-an in am|."hi..xus\uHl is deveh.ped in i.lenti^ally 

 the same way. As I have already puinte.i out. the pin'ment in the 

 eye of the amphioxus lies in the inner end of the cells forming the 

 anterior end of the neural tube. 



In the sturgeon this pigmented area on the inner face of the 



