d up, but in other
forms except _Spirula_ it closes completely and the shell develops
within it. The lateral and posterior borders of the embryo form the
foot, and these borders grow out into ten or eight lobes which become
the arms, and which at first, as seen in fig. 35 (8), are entirely
posterior to the mouth. Development actually shows the anterior arms
gradually growing round the mouth and uniting in front of it. Between
the mantle and the foot are two ridges which form the funnel, and
their position shows them to be the epipodia. The otocysts and eyes
are formed as invaginations of ectoderm, the former behind the eyes,
at the sides of the funnel. All the nerve-centres, cerebral, visceral,
pedal and optic, are formed as proliferations of the ectoderm. At the
sides of the optic ganglia a pair of ectodermic invaginations are
formed, which in the adult become the white bodies of the eyes,
surrounding the optic ganglion. These are vestiges of lateral cerebral
lobes which degenerate in the course of development.

[Illustration: FIG. 35.--Development of _Loligo_.

1. View of the cleavage of the egg during the first formation of
embryonic cells.

2. Lateral view of the egg at a little later stage. a, Limit to
which the layer of cleavage-cells has spread over the egg; b,
portion of the egg (shaded) as yet uncovered by cleavage-cells; ap,
the auto-plasts; kp, cleavage-pole where first cells were formed.

3. Later stage, the limit (a) now extended so as to leave but little
of the egg-surface (b) unenclosed. The eyes (d), mouth (e) and
mantle-sac (u) have appeared.

4. Later stage, anterior surface, the embryo is becoming nipped off
from the yolk-sac (g).

5. View of an embryo similar to (3) from the cleavage-pole or
centro-dorsal area.

6. Later stage, posterior surface.

7. Section in a median dorso-ventral and antero-posterior plane of
an embryo of the same age as (4).

8. View of the anterior face of an older embryo.

9. View of the posterior face of an embryo of the same age as (8).

Letters in (3) to (9):--a, lateral fins of the mantle; b,
mantle-skirt; c, supra-ocular invagination to form the "white body";
d, the eye; e, the mouth; f^1, f^2, f^3, f^4, f^5, the five paired
processes of the fore-foot; g, rhythmically contractile area of the
yolk-sac, which is itself a hernia-like protru