582 PRIMITIVE TRACE : DEVELOPMENT OF VERTEBRAL COLUMN. 
757. The first indication of the permanent fabric in all 
Vertebrated animals, consists of a delicate longitudinal streak, 
termed the “primitive trace” (fig. 316, 6), that is observable 
in the midst of a pellucid area, which is again surrounded by 
a ring of more opaque as¬ 
pect ( c ). This primitive trace 
is the foundation of the 
Yertebral Column. It is in 
the first instance a mere 
furrow in the outer layer of 
the germinal membrane; 
but the sides of this furrow, 
known as the dorsal laminae , 
rise up and arch-over, so as 
gradually to meet and con¬ 
vert the furrow into a canal. 
The meeting first takes place 
in what is afterwards to be¬ 
come the middle of the back; 
and here we find the first 
distinct rudiments of the 
vertebral column, in the 
condition of a series of small square plates (figs. 317, c> c , 
325, Z, l) on either side, which are the representatives of the 
arches of as many vertebrae. The furrow widens-out in the 
situation of the head, so as to form the receptacle (d) for the 
series of large ganglionic masses that is to constitute the brain 
(fig. 323, d, e,f ); and though its sides do not there close-in 
for some time longer, it receives a special hood-like covering 
from a peculiar fold of the germinal membrane, the edge of 
which is seen at e , fig. 317. The cells, of which the parts of these 
laminae that bound the bottom &nd sides of this furrow are 
composed, appear to furnish the rudiments of the nervous 
centres that are afterwards to occupy the canal; but beneath 
its deepest part there lies a continuous rod of peculiar nucleated 
cells (/), the chorda dorsalis , which marks-out the situation 
afterwards to be taken by the bodies of the vertebrae. This 
remains the only representative of the vertebral column in 
the Lamprey and other Fishes of a low grade, the develop¬ 
ment of whose bony skeleton is checked so early that it never 
advances beyond this simple embryonic type (§ 53). 
Fig. 316.— Yolk-Bag of Fowl’s Egg, 
after twelve hours’ incubation : 
t, yolk; b, primitive trace surrounded by 
pellucid area; c, more opaque ring, the 
commencement of the vascular area. 
