2i8 Journal of Comparative Neurology and Psychology. 



Finally, the process by which the primary sense cells are sup- 

 posed to be transformed into ganglion cells is compared to the 

 process in lower invertebrates by which the epithelio-sense cells 

 wander inward to help form the central nervous system. While 

 this again is conceivable it should be noticed that it is quite the op- 

 posite to what we see going on in the vertebrates ; namely, the 

 wandering of cells out from the central nervous system toward 

 the areas of their distribution. 



The writer would suggest that the facts may be equally well 

 explained by supposing that the placodes represent material 

 comparable to the neural crest which has separated more com- 

 pletely from the brain tube than has the remainder of the crest, 

 and that the new formation of neuromast cells is the only 

 peculiar feature of the acustico-lateral system. It is not cer- 

 tainly known what nerve components owe their origin to the 

 neural crest but the writer believes that the general cutaneous 

 component is universally regarded as having that origin and the 

 same assumption is at least tacitly made for the general visceral 

 component. It seems probable from our present knowledge 

 that; (i) in Amphioxus all or the greater part of the material 

 representing the neural crest of vertebrates is included within 

 the brain tube ; (2) in selachians, teleosts, and amphibia at least, a 

 part of the cells whose dendrites supply the skin and which are 

 homologous with the general cutaneous ganglion cells remain 

 within the spinal cord during embryonic and in some cases adult 

 life ; and (3) the neural crest presents all degrees of separation 

 from the neural tube during the development of the embryo. 

 In some selachians (59) it separates from the neural tube by a 

 process of hollow budding, while portions of some of the ganglia 

 are formed of cells which migrate out from the lateral surface of 

 the brain at a comparatively late period. In amphibia the re- 

 lations of the neural crest vary greatly, but in Necturus (108) 

 there is an unusually close connection between the brain and the 

 ectoderm through the neural crest and in the case of the VII 

 ganglion cells cells continue for a long time to migrate from the 

 wall to the ganglion, while the V ganglion is very largely formed 

 by cells coming from the ectoderm. The fact is that there is a 



