510 ALELS. [Vo.. II. 
where they occur; but in young specimens they are found in 
lines or series, connected more or less distinctly in hardened 
specimens by a whitish cord similar to that connecting the pit 
and canal organs at this age. They are first seen in specimens 
of from one to two days old as faint whitish spots adjoining a 
canal line, as shown in Fig. 4, median to the line of the supra- 
orbital canal. This spot soon breaks up into, or is replaced by, 
a series of spots, as shown in the four-day-old specimen repre- 
sented in Fig. 5. In this specimen similar rows are also seen 
on the other side of the supra-orbital and on each side of the 
infra-orbital line, while faint markings on the cheeks, median to 
the squamosal and post-temporal part of infra-orbital, indicate 
still other places where they will soon appear. After this age 
the organs develop rapidly, and in specimens five days old 
(Figs. 6 and 7, Pl. XXXI.) they are already numerous on the 
anterior half of the head. In specimens ten or twelve days old 
(Figs. 8 to 11, Pls. XXXII. and XXXIII.) they appear on the 
operculum ; and when the fish is from three to four weeks old 
(Fig. 13, Pl. XXXIV.), they have spread over the entire head 
and part of the body. In these older specimens the serial 
arrangement of the organs has disappeared, except in those parts 
of the head where they are just beginning to appear. In such 
places (Fig. 9, Pl. XXXII.) they are still found in rows, and 
more or less distinctly connected by a cord. 
6. Innervation. 
It is now well known that all the sense-organs belonging to 
the canals of the lateral system are innervated by dorsal branches 
of the cranial nerves. For these organs Beard has recently pro- 
posed the name branchial sense-organs, because of their develop- 
ment in early embryos from thickenings of the epiblast over 
each branchial cleft, and for the nerves the name suprabranchial 
nerves (No. I, pp. 171 and 174). 
According to his general scheme of development in Elasmo- 
branchs, based largely on the researches of Balfour, Marshall, 
and Van Wijhe, the dorsal root of a cranial nerve grows out- 
ward and downward from the neural crest, toward a local thicken- 
ing of epiblast already formed over the cleft of its segment. 
At the level of the notochord, it fuses with the epiblastic thick- 
