424 L. S. STONE 



In connection with the general cutaneous component of X, 

 Coghill ('16) finds in the 'non-motile' stage an intimate relation 

 between an ectodermal thickening and a loose aggregation of 

 cells upon the lateral aspect of the lateral-line ganghon of X. This 

 cluster of cells, which he identifies as representing the jugular 

 ganglion, is slightly more condensed in its most rostral portion 

 where an incipient root is forming, which does not, however, at 

 this stage reach the brain. 



Lateral-line prirnordia. Piatt ('96), Landacre ('10), Coghill 

 ('16), Goette ('14), and others have shown that a different system 

 of placodes is concerned in the formation of the lateral-line gan- 

 glia of VII, IX, and X. Not all of the investigators agree as to 

 the exclusive derivation of these ganglia from placodes, for some 

 have expressed the belief that neural crest also enters into their 

 formation. 



The primordia of the lateral-line system have been shown very 

 clearly by Piatt ('96), Landacre ('10), and Landacre and Conger 

 ('13) to have origins independent of the auditory vesicle. How- 

 ever, anterior and posterior prolongations of the auditory thick- 

 ening have been observed in many forms, and to these Landacre 

 ('10) applies the term of 'preauditory and postauditory' placodes. 

 He does not have a clear idea as to the relation of these placodes 

 to the sensory lines, for in the case of the preauditory placode 

 (Landacre and Conger, '13) it seemed to disappear by a process of 

 degeneration before the appearance of the lateral-line primordia, 

 and in the case of the postauditory placode ('10) it loses the char- 

 acteristic cell arrangement and does not give rise to the lateral- 

 hne organs. Moreover, in Lepidosteus Landacre and Conger 

 ('13) fail to find any postauditory placode. 



Although separate origins have been given to each of the groups 

 of the lateral-line sense organs by various investigators, none 

 have described such an early pattern of the lateral-line system 

 as the one observed in Necturus. According to Piatt ('96, 

 p. 491), the plan of this system is early laid down in three longitu- 

 dinal lines on each side of the embryo, connected by intersegmen- 

 tal cross-lines with special differentiations at points of intersec- 

 tion, and out of this pattern certain portions are retained in the 



