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THE AMERICAN NATURALIST [Vol. XL 



in response^ to the - demands of the organs of special sense, with 

 which is associated extension of the coordination apparatus. With 

 such additions we have the compound brain of all other known 

 vertebrates up'to man, inclusive." 



"Reasons why the anterior end of the nerve cord of Amphioxus 

 is a brain. It is a brain because : — 



1. It forms the anterior termination of the neural axis. 



2. It stands in intimate relation to the sense organs, eye, and 



3. It gives off at least two pairs of sensory nerves provided with 



peripheral ganglia. 



4. It possesses large groups of ganglion cells forming centers of 



coordination. 



5. It possesses an enlarged section of the central canal in the 



form of an unpaired ventricle with three well marked 

 diverticula — two optic, one olfactory. 



6. It is the largest part of the nervous system, at a time when 



the massive musculature and branchial apparatus of the 

 anterior middle fourth of the body have not reached the 

 stage requiring much enlarged central accommodations. 



7. It shows in young larvje growth to such an extent as to cause 



a ventral flexure of the chorda, while the brain itself bends 

 downwards and so produces a " cranial flexure." 



8. It shows in all other details of structure that it is not simply 



the anterior end of the spinal cord, but a brain. 



9. It shows in a larval stage, soon after the differentiation of 



fibers in the neural axis (larvae with one gill slit), a marked 

 differentiation into ganglionic and fibrous regions, and the 

 boundaries of the unpaired ventricle as well as of the lamina 

 terminalis are distinctly marked out. There is then a ven- 

 tricular segment of the brain reserved for the special sense 

 organs. The fibers appear simultaneously with the forma- 

 tion of the pigment spot, and are in all probability the 

 ways by means of which the sensations from this special 

 sense organ are conveyed backward to the motor centers. 

 10. Since amphioxus is a vertebrate, these relations must have 

 direct and important bearings on the phylogeny of the 

 vertebrate brain and head, and will afford us invaluable 

 aid in clearing up these intricate problems." 



