In the middle of the body we find a thin string of cartilage, which goes the whole length of the body from front to back, and is pointed at both ends (Fig. 210 _i_). This straight, cylindrical rod (somewhat compressed for a time) is the axial rod or the _chorda dorsalis_; in the lancelet this is the only trace of a vertebral column. The chorda develops no further, but retains its original simplicity throughout life. It is enclosed by a firm membrane, the chorda-sheath or _perichorda._ The real features of this and of its dependent formations are best seen in the transverse section of the Amphioxus (Fig. 211). The perichorda forms a cylindrical tube immediately over the chorda, and the central nervous system, the medullary tube, is enclosed in it. This important psychic organ also remains in its simplest shape throughout life, as a cylindrical tube, terminating with almost equal plainness at either end, and enclosing a narrow canal in its thick wall. However, the fore end is a little rounder, and contains a small, almost imperceptible bulbous swelling of the canal. This must be regarded as the beginning of a rudimentary brain. At the foremost end of it there is a small black pigment-spot, a rudimentary eye; and a narrow canal leads to a superficial sense-organ. In the vicinity of this optic spot we find at the left side a small ciliated depression, the single olfactory organ. There is no organ of hearing. This defective development of the higher sense-organs is probably, in the main, not an original feature, but a result of degeneration.