DEVELOPMENT OF ELASMOBRANCH FISHES. 143 



fig. Sh, and 8c). In this position the nerve though small 

 is still very distinct in all good sections, and is formed of a rod 

 of protoplasm, 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 an- 

 teriorly and throughout the greater part of its length, but 

 widens out at its hinder end, and is widest of all at its ter- 

 mination, which is perfectly abrupt. The following measure- 

 ments 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 0*17 Mm. broad. At 

 a point not far from this it was 0-09 Mm. broad, and anteriorly 

 it was 0*05 Mm. broad. These measurements clearly show 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. 0'28 Mm., a 

 breadth which strangely contrasts with the breadth, viz. 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 mucous layer becoming more columnar (PL xil. 

 fig. 3a). 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. xil. fig. 35). Nearer the hind end of the trunk a 

 space appears in it between its columnar cells and the flattened 

 cells of the outermost layer of the skin (PL xii. fig. 3c), and 

 this space becomes posteriorly invested by a very definite layer 

 of cells. The space (PL xii. fig. Sd) or lumen has a slit-Hke 

 section, and is not formed by the closing in of an originally 

 open groove, but by the formation of a cavity in the midst 

 of the cells of the lateral line. Its walls are formed by a layer 

 of columnar cells on the inner side, and flattened cells on the 

 outer side, both layers however appearing to be derived from 



