418 THE EVOLUTION OF MAN. 



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

 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 angle forward along the 

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

 numerous dorsal muscles (Fig. 15, r and 6). 



In the centre of the body is a thin cartilaginous 

 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 

 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, 7?i). 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 thick wall of which 

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

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

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

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

 rudimentary brain. On the foremost end there is also H 

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

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



