June, 1920] Origin of Cerebral Ganglia 307 



double condition appears very early, and there may be two 

 sources for this ganglion instead of one. The one source is 

 unquestionably the auditory vesicle, and the other is somewhat 

 in doubt. Whether this double condition corresponds to the 

 vestibular and cochlear divisions cannot be stated. 



(d) A fourth source of confusion arises in following the 

 dorso-lateral series of placodes. Where the auditory vesicle 

 begins to form, the thickening in the ectoderm and sometimes 

 even the invaginations is much longer than the vesicle itself. 

 The greater portion of the anterior and posterior extensions 

 of the vesicle disintegrate or are incorporated in the vesicle 

 and are concerned in the formation of neither ganglia nor 

 lateral line organs, but their position makes it difficult to 

 distinguish them from ganglion forming dorso-lateral placodes, 

 and sometimes from lateral line organ placodes and even from 

 epibranchial placodes when these form early and the anterior 

 extension of the auditory vesicle extends ventrally in the region 

 of the VII ganglion. 



It must be evident from these facts, that the generalization 

 that acoustico-lateral ganglia come from the dorso-lateral 

 placodes, must be made with the clear understanding of the 

 •several modifying factors that obscure, or, possibly, alter the 

 generalization. The dorso-lateral placodes have been studied 

 more extensively than any other series, except the neural crest, 

 but much of the work was done without a clear conception of 

 the components involved, and was frequently confused by the 

 failure to distinguish between the origin of lateral line ganglia 

 and the origin of lateral line organs, as well as the failure to 

 distinguish the placodes from other thickenings lying adjacent 

 to them in the ectoderm. 



The third of the sources of origin of the cerebral ganglia 

 is the epibranchial placodes. These are much more uniform in 

 their behavior than the dorso-lateral series, and less disturbed 

 by adjacent structures, although sometimes closely related in 

 position to structures resembling placodes, and concerned in gill 

 slit formation, and of a transient nature. It must be admitted 

 that less work has been done and less positive evidence collected 

 as to the ganglion forming activity of the epibranchial placodes 

 than on either of the other two ganglion forming series. This 

 is due to several causes. First, the evidence for this conception 

 is not so easily observed, although convincing enough when 



