264 Journal of Comparative Neurology and Psychology. 



of the primitive neural plate. The material used in the form- 

 ation of these organs was a portion of that part of the neural 

 crest which gave rise to the general cutaneous component. 

 The neuromast area was largest in the region of the VII nerve 

 and neuromere vii, but extended forward probably one or two 

 segments. It can not be exactly compared with the dorsal 

 rami. It is thought that this area, together with the cutaneous 

 ganglion cells, remained outside the neural tube and that later 

 the ganglion cells migrated into the space between the ectoderm 

 and neural tube. This is repeated in the ontogeny in the phe- 

 nomena of the dorso-lateral placodes and the formation of gan- 

 glia from them. 



8. The visual organs are thought to have been derived 

 from the neural crest area also, in segments 2, 3 and 4. The 

 retina of the lateral eye is supposed to contain the equivalent 

 of a general cutaneous ganglion and its primary brain center. 

 The optic tract is homologous with the internal arcuate iibers 

 which arise from the cutaneous centers, decussate in the base of 

 the brain and run to the tectum opticum. The epiphyses are 

 thought of as modified general cutaneous ganglia whose centers 

 have remained in the brain. 



g. The olfactory organ is regarded as a special collection 

 of sense cells of the invertebrate type, sensitive to chemical 

 stimuli, which are gathered above the hypophysial opening as 

 the organs of the same type in the invertebrate are gathered in 

 the roof of the mouth and on the prostomium. 



10. On account of the similarity of their function it is ex- 

 pected that some morphological relation will be found between 

 the olfactory and gustatory organs. The source of the end 

 buds, the origin and history of the nerve components which in- 

 nervate them, and the course of the gustatory paths in the brain 

 constitute one of the most important problems in vertebrate 

 morphology. 



11. It is important that the origin of each of the sensory 

 components in the cranial nerves should be fully worked out in 

 at least a few forms. The cells which enter the ganglia from 

 the neural crest and from the placodes should be traced con- 



