238 Journal of Comparative Neurology and Psychology. 



in the cord and oblongata. Whether the homology is correct 

 or not, it should be noted that the inferior lobes are formed as 

 an expansion of the mid-ventral part of the brain wall, their 

 cavity being the greatly expanded ventral furrow of the third 

 ventricle ventral of the thalamic nucleus of the somatic motor 

 fasciculus. Fig. i shows this relation. An expansion ventro- 

 laterally of this part of the brain wall would very naturally cause a 

 separation of the ventral commissure belonging here into two 

 parts, one running in front and the other behind the inferior 

 lobes. The homology of the tractus lobo-bulbaris is suggested 

 in Fig. 7 by the .symbols used, but the question of this homo- 

 logy does not affect the present argument, since the tractus lobo- 

 bulbaris is present in both decussations. 



The presence of centrifugal fibers from the tectum to the 

 retina should receive explanation in a hypothesis regarding the 

 origin and morphology of the eye. From the study of the cu- 

 taneous centers in lower vertebrates the writer has been led to 

 suggest (69) that these centers primitively possessed a co-ordin- 

 ating apparatus consisting of fibers which arose in the dorsal 

 horn, decussated in the ventral commissure and ended in the 

 dorsal horn of the opposite side at a higher or lower level. It 

 was supposed that these fibers have gradually passed farther 

 forward until they came to form the tractus spino- et bulbo- 

 tectalis which ends in the tectum opticum. The cause of the 

 shifting forward and concentration of this system of fibers would 

 be found in the development in the cord of the tract cells which 

 serve the same function more directly, and in the growth for- 

 ward of the ascending branch of the cutaneous root fibers in the 

 dorsal tracts to the medulla oblongata. Upon this supposition 

 it will be seen that the centrifugal fibers in the optic nerve would 

 represent the fibers passing from the cutaneous center of one 

 side by way ot the ventral commissure to the cutaneous center 

 of the other side in a more cephalic segment ; in this case, from 

 the tectum by way of the chiasma to the retina of the other side. 

 The fact that the centripetal fibers of the optic tract pass back- 

 ward to reach the tectum as a co-ordinating center is explained 

 simply by the fact that the cutaneous center of the only neuro- 



