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 

 Vertebral 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 lamince, 

 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, I, I) on either side, which are the representatives of the 

 arches of as many vertebras. 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 

 lamines that bound the bottom and 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 : 



a, yolk; b, primitive trace surrounded by 

 pellucid area; c, more opaque ring, the 

 commencement of the vascular area. 



