June, 1920] Origin of Cerebral Ganglia 305 



type of receptive and conductive apparatus in the vertebrate 



body. 



The so-called optic ganglion and placode are still more 

 unique, in that their cells are invaginated into the neural tube 

 and, later, evaginated to form the optic cup, thus undergoing 

 two displacements, whereas the olfactory placode undergoes 

 none. 



There is not much satisfaction, while attempting to reduce 

 these phenomena to a system, to have two such striking excep- 

 tions as these organs present. Still, it is better, for the present, 

 in my opinion, to so treat them than to attempt, as is often done, 

 to reduce all sense organs and other thickenings, even the lens, 

 to the three ganglion forming series, and thus weaken the 

 principle involved by doubtful homologies in the attempt to 

 explain every detail. By admitting that we do not see the 

 homology of these placodes with the three ganglion forming 

 series, we at least leave something interesting to be done in the 

 future. Aside from the fact that these placodes come from the 

 ectoderm, they show so little resemblance to each other, or to 

 the three fundamental series, that we cannot combine them or 

 place them in any of our three series. It would be more logical 

 to have five classes: a, neural crest, b, dorso-lateral, c, epi- 

 branchial, d, olfactory, or permanent skin placode, and e, 

 optic, or brain placode. This increases the number of classes, 

 but does not establish any uncertain homologies. If the 

 area that forms the eye had remained in the lateral ectoderm 

 and had formed the eye in that position as it does in some 

 invertebrates, the process would have been homologous to 

 the formation of the olfactory vesicle. It could not even then 

 have been placed in any of the three ganglia forming series, 

 since they form only ganglia which serve as intermediaries 

 between sense organs and brain, while both the optic and 

 olfactory organs contain both receptors and conductors. 



Let us return now to the second of our ganglion forming 

 series, viz., the dorso-lateral placodes. These placodes give 

 rise to the acoustico-lateral ganglia and nerves, and are always 

 situated at or about the level of the auditory vescicle. This 

 series must be sharply distinguished from (1) the profundus 

 thickening or placode, which lies at a somewhat higher level 

 on the side of the body and appears earlier; (2) from the pri- 

 mordia of the lateral line organs, which usually arise much 



