THE SPINAL NERVES IN ELASMOBRANCH FISHES. 
195 
PLATE 18. 
Figs. K i, K H. Two horizontal longitudinal sections through an embryo in which 
the external gills have just appeared. K i is the most dorsal of the two sec- 
tions. Camera. (Zeiss CC ocul. 1.) 
The sections show the relative positions of the anterior and posterior roots 
at different levels. 
pr, posterior nerve-rudiment ; ar, anterior nerve-rudiment ; sp.c, spinal 
cord ; n.c , neural canal ; mp, muscle-plate ; mp', first-formed muscles. 
Fig. L. Longitudinal and vertical section through the trunk of a Scy Ilium-embryo after 
the external gills have attained their full development. Camera. (Zeiss CC 
ocul. 1.) 
The embryo was hardened in a mixture of chromic acid and osmic acid. 
The section shows the commissures which dorsally unite the posterior roots, 
and also the junction of the anterior and posterior roots. The commissures 
are unfortunately not represented in the figure with great accuracy ; their 
outlines are in nature perfectly regular, ancl not, as in the figure, notched at 
the junctions of the cells composing them. Their cells are apparently more 
or less completely fused, and certainly not nearly so clearly marked as in the 
figure. The commissures stain very deeply with the mixture of osmic and 
chromic acid, and form one of the most conspicuous features in successful 
longitudinal sections of embryos so hardened. In sections hardened with 
chromic acid only they cannot be seen with the same facility. 
sp.c, spinal cord ; cjr, grey matter ; w, white matter ; ar, anterior root ; 
p>r, posterior root ; x , commissure uniting the posterior roots. 
Figs. Mi, Mil. Two sections through the head of the same embryo as fig. B. Mi, the 
foremost of the two, passes through the anterior part of the thickening of 
epiblast, which becomes involuted as the auditory vesicle. It contains the 
rudiment of the seventh nerve, VII. Camera. (Zeiss CC ocul. 2.) 
VII, rudiment of seventh nerve ; au , thickening of external epiblast, which 
becomes involuted as the auditory vesicle ; n.c , neural canal ; ch , notochord ; 
pp, body-cavity in the head ; so, somatopleure ; sp, splanchnopleure ; al, 
throat, exhibiting an outgrowth to form the first visceral cleft. 
2 E 
MDCCCLXXVI 
