No. 2.] VERTEBRATE CEFHATOCENESIS. 22$ 



present condition of morphology and physiology. Stcincr has 

 shown, by a beautiful series of experiments on numerous forms, 

 that we may only speak of a brain from the physiologist's stand- 

 point when we have associated with the general centre of loco- 

 motion one or more of the organs of the higher senses, and this 

 definition is entirely admissible from a morphological standpoint. 

 While Steiner's experiments do not show the presence of a 

 brain in Amphioxus, they do not, I think, in any way demon- 

 strate or even render probable its absence, and it can be shown 

 that the morphological or physical basis required by Steiner's 

 definition is present. 



I would define the vertebrate brain as follows : The ''vertebrate 

 brain " is that portion of the anterior part of the axial nerve cord, 

 associated ivith organs of special sense, containing an enlargement 

 of the central canal which is carried out into all structures formed 

 by the outgrowth of the brain wall. Its zvalls cotitain the principal 

 centres for the co-ordination of sensations and movements. All 

 further additions to this simple brain {Amphioxus) are made in 

 response to the demands of the organs of special sense, ivith which 

 is associated extensioft of the co-ordination apparatus. With such 

 addittotts we have the compound brain of all other knozvn verte- 

 brates up to man inclusive. 



Reasons why the anterior end of the nerve cord of Amphi- 

 oxus 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 

 nose. 



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

 with peripheral ganglia. 



4. It possesses large groups of ganglion cells forming centres 

 of co-ordination. 



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

 form of an unpaired ventricle with three well marked divertic- 

 ula, 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 has not reached the stage requiring 

 much enlarged central accommodations.^ 



1 When, however, the innervation of the locomotive apparatus (which in the 

 adult animal more than equals the remaining organs in bulk) is fully developed, the 



