ORIGIN OF THE LATERAL LINE PRIMOllDIA 579 



portion, found in the region of the ninth nerve, later forms the 

 lateral line posterior to the auditory vesicle. 



Wilson ('91) in his paper on Serranus, traces the lateral lines 

 and the auditory organ back to a common sensory furrow, which 

 (p. 244) gives rise not only to the ear, but to a ''functional bran- 

 chial sense organ," and to the organs of the lateral line as well. 

 This furrow begins to deepen at two points, where the auditory 

 organ and the branchial sense organ will be formed. The proc- 

 ess of invagination continues until there are two well-marked 

 sacs, the furrow persisting between them and continuing for 

 some distance behind the auditory sac. At a still later stage, 

 the three derivatives (branchial sense organ, auditory sac, and 

 lateral line primordium) have moved away from each other. 

 The (first) gill-slit breaks through and just in front of it is the 

 branchial sense organ, the auditory sac is overgrown by the 

 medulla, and the primordium of the lateral line moves back some 

 distance from its original position in front of the somites. Later, 

 ''the lateral line anlage has grown still farther back, and is 

 incompletely divided into three sense organs of the lateral line." 



Concerning the question at issue, that is, the relation of the 

 lateral line primordia to the auditory vesicle, Wilson is very defi- 

 nite. It is unfortunate, however, that he perpetuated Beard's 

 term "branchial sense organ." Beard thought it was a part of 

 the lateral line system but concerned in some way with the func- 

 tion of the gills. Since Wilson goes no further than to charac- 

 terize it as a 'functional' branchial sense organ, we are left to 

 choose between lateral line pi^imordia, epibranchial placodes, 

 thickenings at the point of contact of endoderm of gill with ecto- 

 derm, and preauditory placode (Landacre '10). The last seems 

 the more probable interpretation and, if correct, the term 

 branchial sense organ should be dropped. 



Wilson and Mattocks ('97) confirm in the salmon the previous 

 work of Wilson in the sea bass, as to the common primordium 

 of the lateral sense organs and the auditory organ, and further 

 state that the portion in front of the auditory vesicle (branchial 

 sense organ of Beard) gives rise by bifurcation to the supra- 

 orbital and infra-orbital lines, while the portion posterior to the 



