THE LATERAL LINE. 363 



linear thickening of the inner row of cells of the external epi- 

 blast, on each side, at the level of the notochord. 



This thickening, in my youngest embryo in which it is found, 

 has but a very small longitudinal extension, being present 

 through about 10 thin sections in the last part of the head "and 

 first part of the trunk. The thickening, though short, is very 

 broad, measuring about O'28 Mm. in transverse section, and 

 presents no signs of a commencing differentiation of nervous 

 structures. The large intestinal branch of the vagus can be 

 seen in all the anterior sections in close proximity to this line, 

 and appears to me to give off to it posteriorly a small special 

 branch which can be traced through a few sections, vide PI. 1 3, 

 fig. 2 n.l. But this branch is not sufficiently well marked to 

 enable me to be certain of its real character. In any case the 

 posterior part of the lateral line is absolutely without any ad- 

 joining nervous structures or traces of stick. 



The rudiment of the epidermic part of the lateral line is 

 formed of specially elongated cells of the mucous layer of the 

 epiblast, but around the bases of these certain rounder cells of a 

 somewhat curious appearance are intercalated. 



There is between this and my next youngest embryo an 

 unfortunately large gap with reference to the lateral line, 

 although in almost every other respect the two embryos might 

 be regarded as belonging to the same stage. The lateral line 

 in the older embryo extends from the hind part of the head to a 

 point well behind the anus, and is accompanied by a nerve for 

 at least two-thirds of its length. 



In the foremost section in which it appears the intestinal 

 branch of the vagus is situated not far from it, and may be seen 

 at intervals giving off branches to it. There is no sign that these 

 are otherwise than perfectly normal branches of the vagus. 

 Near the level of the last visceral cleft the intestinal branch of 

 the vagus gives off a fair-sized branch, which from the first 

 occupies a position close to the lateral line though well within 

 the mesoblast (PL 13, fig. 3^, ;/./). This branch is the lateral 

 nerve, and though somewhat larger, is otherwise much like the 

 nerve I fancied I could see originating from the intestinal branch 

 of the vagus during the previous stage. 



It rapidly thins out posteriorly and also approaches closer 



24 2 



