June, 1920] Origin of Cerebral Ganglia 303 



In the second mode of behavior as seen in Ameiurus, the 

 homologue of the neural crest remains in the lateral ectoderm, 

 and is not located in the re-entering angle. From its position 

 in the lateral ectoderm, it becomes detached finally and migrates 

 to the usual position of neural crest ganglia, after which time 

 there is nothing to indicate that its early behavior was different 

 from the neural crest in the spinal cord region. Since, however, 

 there are other ganglion forming structures in the lateral 

 ectoderm, this mode of behavior raises difficulties not exper- 

 ienced in the study of the first type. 



A third type of behavior is illustrated by the Urodeles, and, 

 according to Tilney, by certain of the cerebral ganglia in the 

 cat. In this mode of behavior, the neural crest is incorporated 

 in the neural tube and completely detached from surface ecto- 

 derm. It is later erupted from the dorsal surface of the neural 

 canal, and assumes the usual position of neural crest ganglia, 

 after which there is nothing to indicate that its mode of origin 

 is different from that of the neural crest in the cord. 



These three types of neural crest behavior can be reduced to 

 one type, theoretically, at least. An extremely broad neural 

 plate would, before invaginating to form the neural canal, reach 

 so far laterally as to incorporate the neural crest in the neural 

 tube from which, later, it is erupted (type III). At the other 

 extreme would be the very narrow neural plate, which would 

 leave the neural crest completely stranded in the ectoderm, as 

 in Ameirus (type II). The intermediate stage would have a 

 moderately broad neural plate, which would carry the neural 

 crest as far mesially as the re-entering angle (type I). 



The ganglia derived exclusively from the neural crest in the 

 head are, Gasserian of V, the jugular or root gangHon of IX 

 when present, and the jugular or root ganglion of X. All 

 of these belong to the general somatic or tactile ganglia. The 

 neural crest further furnishes . the general visceral ganglia 

 supplying apparently free nerve endings in the mucosa. These 

 are found in the geniculate, nodosal and petrosal ganglia, but 

 do not form all of these ganglia, since they contain special 

 visceral or gustatory ganglia derived from the epibranchial 

 placodes. 



The only ganglion of the general somatic or general visceral 

 series that deviates from this description in any noticeable 

 degree is the profundus ganglion. This ganglion always 



