Johnston, Morphology of the Head. 221 



or completely interrupted at two points. We may imagine, 

 then, that when the neural plate rolled up and sank beneath the 

 surface, the neuromast area was left outside ; and we may find 

 the cause for its failure to be included, as the rest of the neural 

 crest was, in the fact that the special sense cells were already 

 developed in this area. If now we consider that the primitive 

 cutaneous sense cells remained in the neuromast area, as their 

 absence from the neural crest of this region indicates, we have 

 all the essential relations of the neuromast organs accounted for. 

 The further development consisted in the sinking in of the area 

 to form a pit for the betcer protection of the sense cells, or for 

 their better functioning, or both, and the migration of the 

 ganglion cells into the space between the infolded neural tube 

 and the auditory pit, where they found more room. The de- 

 pressed area became the forerunner of the auditory vesicle and 

 the head and trunk lines of neuromast organs. The general 

 cutaneous sensory cells of the area acquired permanent relations 

 with the newly formed sense cells, and their migration inward to 

 form the ganglia of the lateral line nerves has become a per- 

 manent feature of vertebrate development. 



The extent of the neuromast area has thus far been indi- 

 cated only in the most general way for the double reason that 

 we should not expect it to be sharply circumscribed at least in 

 the beginning, and that there are reasons for thinking that it 

 extended farther forward than is generally supposed. The 

 whole of the acustico-lateral system is described as arising in 

 some selachians (93, i25)andteleosts(i2 i, 122) from a single some- 

 what extensive thickening of ectoderm from which the two head 

 lines grow forward and the trunk lines grow backward. In 

 cyclostomes (79, "j^), however, there are two large and separ- 

 ate placodes, in front of the acustico-facialis placode, with which 

 the profundus and trigeminal ganglia come into contact. One 

 would think from the relations that these two placodes, which 

 give rise to the ophthalmicus superficialis facialis and buccalis 

 nerves, belonged originally to the profundus and trigeminal seg- 

 ments. This idea is strengthened by the fact that in the 

 Gymnophiona (11) a placode appears in the same situation 



