No. 2.] VERTEBRATE CEPHALOGENES/S. 22/ 



mesocoele of higher forms. The cerebellum is simply a dorsal 

 commissure of the medullary region, as has been shown by 

 Osborne and others from studies on higher vertebrates. I have 

 found no trace of a cerebellar commissure in Amphioxus. The 

 open ventricle of the medulla is produced by the great increase 

 of ganglion cells, and especially of the fibre tracts which must 

 of necessity pass through this region in order to reach the cen- 

 tres of co-ordination in the parts anterior to it. The basal and 

 lateral portions of its walls are thus greatly thickened, the gan- 

 glion cells of the nerve nuclei arrange themselves in harmony 

 with these changed conditions, and the dorsal wall is greatly 

 extended without at the same time being structurally compli- 

 cated. The medulla of higher vertebrates is not co-extensive in 

 all forms and includes a varying number of the segments repre- 

 sented in Amphioxus by simple spinal segments. 



The brain of Amphioxus possesses all those functions which 

 in higher vertebrates are possessed by thalamencephalon mid- 

 brain, and medulla, except an undetermined number of the pos- 

 terior segments of the medulla of the higher forms. Of course 

 these functions are all of a milder nature than in the higher 

 animals. 



The reason for Steiner's failure to find a medullary centre of 

 co-ordination lies in the intense nature of the stimuli applied to 

 the nervous apparatus (resection, etc.) and to the small size of 

 the region to which the stimuli must be applied. The nervous 

 apparatus is of such a delicate nature that such strong stimuli 

 serve to call forth the activities of each segmental centre of 

 locomotion, and prevent the observation of any other results of 

 stimulation. 



Much more delicate methods must be used to obtain reaction 

 of the organs of special sense. Of the possible means of ex- 

 perimentation, which appear to me likely to give good results in 

 the case of the pigment spot and the olfactive pit, are circum- 

 scribed application of bundles of light rays of varying intensity and 

 wave lengths, and the application of olfactive stimuli by means 

 of fine capillary tubes to the resting animals in sea-water. 



C. The ontogenetic changes of the neural axis in other verte- 

 brates carries the brain through the condition zvJiich in Amphi- 

 oxus retnains permanent as the adtilt brain. 



