STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 
367 
backwards for about half tbe length of tbe embryo. Tbe tail has much the same 
appearance as in tbe last stage. 
Tbe vertebral regions of tbe mesoblastic plates are now segmented for tbe greater part 
of the length of the trunk, and tbe somites of which they are composed (Plate 23, 
fig. 30, pr.) are very conspicuous in surface views. 
Our sections of this stage are not so complete as could be desired : they show, how¬ 
ever, several points of interest. 
Tbe central canal of tbe nervous system is large, with well-defined walls, and in 
hardened specimens is filled with a coaguhun. It extends nearly to tbe region of tbe 
tail. 
The optic vesicles, which are so conspicuous in surface views, appear in section 
(Plate 22, fig. 26, op.) as knob-like outgrowths of the fore-brain, and very closely 
resemble the figures given by Oellacher of these vesicles in TeleosteiV 
From tbe analogy of tbe previous stage, we are inclined to think that they have a 
lumen continuous with that of the fore-brain. In our only section through them, 
however, they are solid, but this is probably due to the section merely passing through 
them to one side. 
The auditory pits (Plate 22, fig. 27, cm.) are now well marked, and have the form of 
somewhat elongated grooves, the walls of which are formed of a single layer of 
columnar cells belonging to the nervous layer of the epidermis, and extending inwards 
so as nearly to touch the brain. 
In an earlier stage it was pointed out that the dorsal part of the medullary keel 
was different in its structure from the remainder, and that it was destined to give rise 
to the nerves. The process of differentiation is now to a great extent completed, and 
may best be seen in the auditory region (Plate 22, fig. 27, VIII.). In this region 
there was present during the last stage a great rhomboidal mass of cells at the dorsal 
region of the brain (Plate 22, fig. 24, VIII.). In the present stage, this, which is 
the rudiment of the seventh' and auditory nerves, is seen growing down on each side 
from the roof of the hind-brain, between the brain and the auditory involution, and 
abutting against the wall of the latter. 
Fudiments of the spinal nerves are also seen at intervals as projections from the 
dorsal angles of the spinal cord (Plate 23, fig. 29, sp.n.). They extend only for a short 
distance outwards, gradually tapering off to a point, and situated between the epiblast 
and the dorsal angles of the mesoblastic somites. 
The process of formation of the cranial nerves and dorsal roots of the spinal nerves is, 
it will be seen, essentially the same as that already known in the case of Elasmobranchii, 
Aves, &c. The nerves arise as outgrowths of a special crest of cells, the neural crest 
of Marshall, which is placed along the dorsal angle of the cord. The peculiar posi¬ 
tion of the dorsal roots of the spinal nerves is also very similar to what has been met 
* “Beitrage zur Entwick. d. Knochenfische,” Zeit. f. wiss. Zook, vok xxiii., 1873, taf. iii., fig. ix., 2. 
MDGCCLXXXII. 3 B 
