364 DEVELOPMENT OF ELASMOBRANCH FISHES. 



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 (PI. 13, figs. 3^, 

 and 3r). In this position the nerve though small is still very 

 distinct in all good sections, and is formed of a rod of pro- 

 toplasm, with scattered nuclei, in which I could not detect a 

 distinct indication of cell-areas. The hinder part of the nerve 

 becomes continually smaller and smaller, without however pre- 

 senting any indication of becoming fused with the epiblast, and 

 eventually ceases to be visible some considerable distance in 

 front of the posterior end of the lateral line. 



The lateral line itself presents some points of not incon- 

 siderable interest. In the first place, it is very narrow anteriorly 

 and throughout the greater part of its length, but widens out at 

 its hinder end, and is widest of all at its termination, which is 

 perfectly abrupt. The following measurements of it were taken 

 from an embryo belonging to stage L, which though not quite 

 my second youngest embryo is only slightly older. At its 

 hinder end it was O'l/ Mm. broad. At a point not far from this 

 it was O'OQ Mm. broad, and anteriorly it was 0x35 Mm. broad. 

 These measurements clearly shew that the lateral line is broadest 

 at what may be called its growing-point, a fact which explains its 

 extraordinary breadth in the anterior part of the body at my first 

 stage, viz. O'28 Mm., a breadth which strangely contrasts with the 

 breadth, viz. 0*05 Mm., which it has in the same part of the body 

 at the present stage. 



It still continues to form a linear area of modified epidermis, 

 and has no segmental characters. Anteriorly it is formed by the 

 cells of the mucous layer becoming more columnar (PI. 13 fig. 

 3). In its middle region the cells of the mucous layer in it are 

 still simply elongated, but, as has been said above, have a gable- 

 like arrangement, so as partially to enclose the nerve (PI. 13, fig. 

 3^). Nearer the hind end of the trunk a space appears in it 

 between its columnar cells and the flattened cells of the outer- 

 most layer of the skin (PI. 13, fig. 3^), and this space becomes 

 posteriorly invested by a very definite layer of cells. The space 

 (Pi. 13, fig. 3<^) or lumen has a slit-like section, and is not 

 formed by the closing in of an originally open groove, but by 



