ORIGIN OF THE LATERAL LINE PRIMORDIA 605 



the lateralis ganglion of X, just as the more anterior is associated 

 with the lateralis ganglion of IX. 



The ectodermic gill thickenings, as mentioned above, are four 

 in number at this stage, three of them lying behind the first 

 three true gills and the fourth lying chiefly in the area of contact 

 of the fourth gill. Since the lateral line primordia are present, 

 it will not be necessary to follow these thickenings further than 

 to mention the fact that the fourth behaves just as the other 

 three, that is, forms an extension behind the fourth true gill and 

 later a fifth is formed in a manner quite similar to the other four. 

 It may be of interest here to recall the fact shown by Landacre 

 ('12) that the epibranchial placodes of the five gills associated 

 with the ninth and tenth nerves can be positively identified first 

 at the following ages; the epibranchial placode of the first gill 

 in Stage XXII; of the second and third gills in Stage XXVI. 



Of the three lateral line primordia present in this series the 

 more anterior occupies relatively the same position as in the pre- 

 ceding series with the exception that, owing apparently to a 

 change in the shape of the body, it is situated somewhat more 

 ventrally with reference to the auditory vesicle. It has increased 

 in size and lies as before at the level of the posterior border of 

 the auditory vesicle. The posterior end of the lateral line pri- 

 mordium extends slightly beyond the posterior end of the audi- 

 tory vesicle (fig. 5). It lies approximately parallel to the thick- 

 ening of the first true gill and is about one-half as long as that 

 thickening. Its anterior end lies at the posterior base of the 

 opercular fold which is first distinguishable in this series, and it 

 extends from this point backwards over the thickening of the first 

 true gill and at a considerably higher level (fig. 32). At its pos- 

 terior end it comes closely into contact with the lateralis ganglion 

 of the IX nerve. 



The condition here, as well as the relation of the lateralis X 

 ganglion to the lateral line primordium, suggests strongly that 

 these ganglia may be derived from the same primordia that give 

 rise to the lateral hne organs in these regions respectively. How- 

 ever, our series are not taken at sufficiently close intervals to 



