14)2 THE LATERAL NERVE, 



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 0*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. XIL 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 adjoining nervous structures or traces of such. 



The rudiment of the epidermic pa.rt 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 he 

 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 in- 

 testinal 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 (PI. XIL fig. 3a, n.l). 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 apioroaches closer 

 and closer to the lateral line. At the front end of the trunk 

 it is quite in contact with it, and a short way behind this region 

 the cells of the lateral line arrange themselves in a gable-like 

 form, in the angle of which the nerve is situated (PL xii. 



