304 The Ohio Journal of Science [Vol. XX, No. 8, 



becomes more or less closely associated with the Gasserian, 

 but it always arises from the lateral ectoderm and, curiously, 

 sometimes so far ventrally on the side of the head that its 

 thickening, or placode, has been taken frequently for a dorso- 

 lateral placode, and consequently the 5th nerve has been some- 

 times described as having a vestigeal dorso-lateral placode, 

 or lateral line organ. This is, of course, incorrect. 



The profundus ganglion and nerve have none of the char- 

 acteristics of lateral line nerves, but do have the characteristics 

 of general somatic or tactile nerves. The mode of origin of the 

 profundus ganglion seems to be constant, regardless of the way 

 in which the neural crest behaves throughout the remainder of 

 the head; that is, whichever of the three modes of behavior 

 the neural crest follows, the profundus ganglion always arises 

 from the lateral ectoderm, somewhat dorsal in position, however, 

 to the dorso-lateral placode to be described later. It is best, in 

 my opinion, to consider the source of the profundus ganglion 

 as the most marked and constant displacement of the neural 

 crest in the head region. This will lead to fewer complications 

 than any other conception. 



In addition to the neural crest in the head, there are two 

 other general sources of cerebral ganglia, namely, the dorso- 

 lateral placodes, giving rise to the acoustico-lateral, or special 

 somatic ganglia and nerves, and the epibranchial placodes, 

 giving rise to the gustatory, or special visceral ganglia and 

 nerves. 



Before taking these up in detail, however, it is advisable 

 to call attention to the behavior of the ectoderm in forming the 

 olfactory and optic placodes, so-called. Nothing is to be 

 gained, in the present state of our knowledge, by attempting 

 to place the placodes of these two organs in the neural crest, or 

 dorsolateral or epibranchial series. The olfactory placodes 

 remain in the ectoderm at the ventrolateral portion of the 

 neural tube primitively, and are unique among the vertebrates 

 in retaining their position in the ectoderm. They certainly 

 have no resemblance to a lateral line placode or ganglion, and 

 their detachment from the gill slit, at least in existing verte- 

 brates, prevents us from homologizing them with the epi- 

 branchial series. Their origin is as unique as their anatomical 

 structure in that they remain in the ectoderm and as compared 

 with other nerves they are usually considered the most primitive 



