132 THE BRAIN OF THE TIGER SALAMANDER 



The embryological studies just mentioned were based on series of 

 normal embryos. The conclusions reached have been checked experi- 

 mentally, extended, and in some details corrected by Stone ('22, '26) 

 and by Yntema ('37, '43), so that we now have very accurate infor- 

 mation about the sources of the nerve cells of each sensory com- 

 ponent of the ganglia of the V to X cranial nerves. The ganglion of 

 the trigeminus is derived chiefly from neural crest, which also con- 

 tributes some cells to the ganglia of the VII, IX, and X nerves 

 (Landacre, '21, p. 15). Yntema ('37) found no neurons of neural- 

 crest origin in the facial ganglion ; but, since there is a small general 

 cutaneous component of this nerve in adult Amblystoma, it is prob- 

 able that some cells of neural-crest origin are present, as is known to 

 be the case in some other animals. Part of the trigeminal ganglion 

 (profundus ganglion of Landacre, '21, p. 23) is delaminated from the 

 lateral ectoderm. The lateral-line ganglia are derived exclusively 

 from the dorsolateral placodes of the ectoderm, and the ganglion of 

 the VIII nerve from the auditory vesicle. The visceral ganglia of the 

 VII, IX, and X nerves arise from epibranchial placodes. Landacre 

 derived only the special visceral (gustatory) component of these 

 ganglia from these placodes, but Yntema has shown that the larger 

 part of the general visceral component also is of placodal origin. Ac- 

 cording to Yntema's analysis, epibranchial placodes give rise to gen- 

 eral and special visceral components of the cranial ganglia, dorso- 

 lateral placodes to lateral-line components, the auditory placode to 

 the VIII ganglion, and neural crest to general cutaneous and general 

 visceral components. The mesencephalic nucleus of the V nerve is 

 derived chiefly from a portion of the neural crest which is incorpo- 

 rated within the neural tube, though it is not certain that this is the 

 exclusive source of these cells (p. 141, and Piatt, '45). 



SURVEY OF THE FUNCTIONAL SYSTEMS 

 GENERAL SOMATIC SENSORY NERVE ROOTS 



Here are included cutaneous sensibility of several modalities — 

 touch, temperature, pain, and, in aquatic animals, refined chemical 

 sensitivity. Associated with these nerves are those of deep pressure. 

 The nervous apparatus of these various qualities of sense has not 

 been successfully analyzed in lower vertebrates. Their fibers are 

 mingled peripherally and also centrally, except for those of the 

 mesencephalic V root. It is not improbable that some peripheral 



