226 HOWARD AVERS. [Vol. IV. 



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

 cause a ventral flexure of the chorda, the brain itself bending 

 downwards, thus producing a "cranial flexure." 



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

 ply the anterior etid of the spi7ial chord, but a brain. 



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

 of fibres 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 the lamina ter- 

 minalis are distinctly marked out. There is then a ventricular 

 segment of the brain reserved for the special sense organs. The 

 fibres appear simultaneously with the formation of the pigment 

 spot, and are in all probability the ways by means of which the 

 sensations from this special sense organ are conveyed backwards 

 to the motor centres. 



10. Since AmpJiioxns is a vertebrate, these relations must Jiave 

 direct and important bearings on the pJiylogcny of the vertebrate 

 braiji and head, and will afford tis itivaliiable aid in clearing up 

 these intricate problems. 



B. The large collections of ganglion cells just posterior to the 

 thalamoccelc are homologous with the medullary nuclei of other 

 vertebrates, since their connections shozv them to be centres for the 

 control of the branchial apparatus, and the sensory a^id motor 

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

 of respiration, deglutition, etc. 



These groups of large ganglion cells do not reach quite to the 

 thalamocosle. A portion of the wall bounding the ventricle 

 posteriorly is free from them, and is made up of such cells as 

 are strictly comparable with the cellular elements of the mid- 

 brain of higher forms. From this narrow territory and the 

 central canal contained in it is derived the mesencephalon and 



brain is relatively smaller, but it still has its peculiar structure. We have other in- 

 stances of the actual preponderance in size of circumscribed tracts of the nervous 

 axis over the brain. In those ancient forms, e.g. Stegosaurus, whose remains have 

 been described by Marsh and others, where the sacral canal is shown to greatly 

 exceed the cranial cavity in its cubic dimensions. But here there is a doubt as to 

 whether the nervous axis in the sacral canal really filled the space, or whether it bore 

 proportionately the same relations to its cavity that the brain did to its cranial space. 

 There is reason to seriously doubt that the sacral enlargement of the cord was actually 

 larger and more important than the brain in relation to the locomotive mechanism of 

 the animal. 



