4l8 THE EVOLUTION OF MAN. i 



underneath into a short, anal fin. The fin-like ridge is sup 1 



ported by a great number of small and delicate quadrangular 

 plates (Plate XI. 15,/). The delicate parallel lines under ; 



the skin, which describe an acute anoxic forward alonof the 

 central line of each side, are the boundary lines of the ; 



numerous dorsal muscles (Fig. 15, r and h). ^ 



In the centre of the body is a thin cartilaginous i 



cord, which traverses the longitudinal axis of the entire body \ 



from front to rear, and is symmetrically sharpened at both  



ends (Fig. 151, i). This is the notochord (chorda dorsalis), ] 

 which in this case takes the place of the backbone, or 

 vertebral column. In the Amphioxus the notochord does not ; 



develop further, but remains permanently in this most simple ' 



original condition. It is enclosed in a firm membranous | 



covering, the notochord-sheath. The nature of the latter, ! 



and of the formations which proceed from it, may be best 

 seen in the transverse section of the Amphioxus (Fig. 152; 

 Plate X. Fig. 13, cs). Immediately above the chorda the I 



notochord-sheath forms a cylindrical tube, and in this tube 

 the central nervous system lies enclosed, the spinal or me- . ' 

 dullary tube (Plate XI. Fig. 15, m). This important mental , 



organ retains throughout life this most simple form, that of 

 a cylindrical tube, the anterior and posterior ends of which 

 are almost equally simple, and the tliick wall of whicli 

 encloses a narrow canal. Tlie anterior end is, indeed, rather 

 rounder, and contains a small, hardly noticeable, bladder- ^ 



like swelling of the canal (Fig. 15, rrij^). This may be re ' 



garded as the first indication of a real brain-bladder ; as :\ 

 rudimentary brain. On the foremost end there is also t\ 

 little black pigment-spot, the rudiment of an eye. Keav ; 



tliis eye-spot, on the left side, there is a little ciliated groove, I 



