ORIGIN OF THE LATERAL LINE PRIMORDIA . 615 



The two remaining primordia appear later than the one just 

 described and are more posterior in position. Both appear for 

 the first time in the same series, the more anterior being located 

 over the second gill thickening and the more posterior extending 

 from a point over the third gill thickening back over the fourth 

 and on over the position of the unformed fifth gill thickening, 

 ending some distance behind this point with its posterior end at 

 a somewhat lower level than its anterior end. 



The more anterior of these two thickenings, or the second, has 

 the same general relations to the second gill thickening that the 

 first lateral line primordium has to the first gill thickening. It 

 is more dorsal in position and has, soon after its appearance, 

 definite anterior and posterior limits and also soon shows the 

 characteristic radial arrangement of cells which is so usual in 

 lateral line primordia. 



Its relation to the lateralis X ganglion is quite similar to the 

 relation of the first lateral line primordium to the lateralis IX 

 ganglion, that is, it is closely in contact with the ganglion at its 

 posterior end and suggests the origin of this ganglion from the 

 primordium of the lateral line. The anterior end of the lateralis 

 X ganglion appears further to be distinct from the longer pos- 

 terior portion, indicating the presence of two lateral line ganglia 

 on X. The lateral line organs differentiated in this primordium 

 receive their fibers from the ramus supratemporalis X nerve. 



Since both the second and third lateral line primordia appear 

 in the same series, a question might be raised as to whether the 

 second might not have been found connected with the third in a 

 closer series. In view of the number of separate lateral line 

 primordia in the preauditory region and in view of the fact that the 

 first postauditory primordium is certainly not connected with 

 the second and third, the question is not of vital significance. 

 The facts mentioned above, in addition to the amount of 

 undifferentiated ectoderm lying between the second and third 

 primordia, are evidence against their continuity in earlier stages. 



The third postauditory lateral line primordium at the time of 

 its appearance is the largest of the three and grows backward 

 rapidly along the side of the body, and on it differentiate the or- 



