neither vertebrae nor skull nor limbs (Figures 1.83 to 1.86). And even
after the formation of the primitive vertebrae has begun, the
segmented foetus of the amniotes still has for a long time the simple
form of a lyre-shaped disk or a sandal, without limbs or extremities.
When we compare this embryonic condition, the sandal-shaped foetus,
with the developed lancelet, we may say that the amphioxus is, in a
certain sense, a permanent sandal-embryo, or a permanent embryonic
form of the Acrania; it never rises above a low grade of development
which we have long since passed.

The fully-developed lancelet (Figure 2.210) is about two inches long,
is colourless or of a light red tint, and has the shape of a narrow
lancet-formed leaf. The body is pointed at both ends, but much
compressed at the sides. There is no trace of limbs. The outer skin is
very thin and delicate, naked, transparent, and composed of two
different layers, a simple external stratum of cells, the epidermis,
and a thin underlying cutis-layer. Along the middle line of the back
runs a narrow fin-fringe which expands behind into an oval tail-fin,
and is continued below in a short anus-fin. The fin-fringe is
supported by a number of square elastic fin-plates.

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 (Figure 2.210 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