ON THE NOTOCHORD 



243 



growth and development, or evolution, of the human 

 organism ; it is a structure, therefore, of an ephemeral 

 order, which, when its developmental services are no 

 longer required, is relegated to that class of " survivals," 

 which now has assumed, a more or /ess, " recognised 



FIG. 102. 



FIG. 101. CERVICAL PART OF THE PRIMITIVE VERTEBRAL COLUMN AND 

 ADJACENT PARTS OF AN EMBRYO CHICK OF THE SIXTH DAY, SHOWING 



THE DIVISION OF THE VERTEBRAL SEGMENTS. (From Kolliket after 



Remak.) 



i. i, chorda dorsalis in its sheath, pointed at its upper end ; 2, points by three lines 

 to the original intervals of the primitive vertebrae ; 3, in a similar manner indi- 

 cates the places of new division into permanent bodies of vertebrae ; c, indicates 

 the body of the first cervical vertebra ; in this and the next the primitive division 

 has nearly disappeared, as also in the two lowest represented, viz. d, and the one 

 above ; in those intermediate the line of division is shown ; 4, points in three 

 places to the vertebral arches ; and 5, similarly to three commencing ganglia oi 

 the spinal nerves : the dotted segments outside these parts are the muscular plates. 



FIG. 102. TRANSVERSE SECTION OF EARLY EMBRYO OF PRISTIURUS 

 (ELASMOBRANCHS). (From Balfour.) 



nc, neural canal ; pr, posterior root of spinal nerve ; x, subnotochordal rod ; ao, aorta ; 

 sc, parietal mesoblast ; sp, visceral mesoblast ; tup, muscle-plate ; mf ', portion of 

 muscle-plate converted into muscle ; Vv, portion of the vertebral plate which will 

 give rise to the vertebral bodies ; al, alimentary canal. 



position," in our scientific structural classifications and 

 nomenclature. 



From its position, as to time of appearance as a fully- 

 formed anatomical element (Figs. 101, 102, 103), amid 

 the teeming array of rapidly evolving structures, and 

 functions, we would " naturally " suppose that it must 

 have a very important duty, or bearing, as well as, from 

 its central, and evidently determining, localisation, on the 



