Savory - Arachnida 1977

46 II. DE ARACH~IDIS
As a rule the first division of the egg is meridional, producing anterior
and posterior cells, a slight flattening of which indicates the future
ventral surface. The second division is also meridional but at right
angles to the first, and the third is equatorial. Thus an eight-celled
consisting of four dorsal and four ventral cells is reached. The irregular
dividing of the cells which now follows is accompanied by a movement
of the cells between the yolk masses to the periphery, so that ultimately
the embryo consists of perhaps 100 cells surrounding the yolk within.
This stage may be called the periblastula.
The process of gastulation is represented by a multiplication of some
of the cells of the ventral surface, where, at a point near the anterior
end there appears an opaque, slightly projecting mass. This is called the
anterior cumulus; its cells continue to divide, absorbing yolk as they do
so, and ultimately come to form the mesoblast and hypoblast. In some
orders a posterior cumulus arises at the same time and in the same way,
in others the anterior cumulus divides into two. The subsequent meeting
of the two cumuli or the division of the only one is the first distinction
between the prosoma and opisthosoma of the future (Fig. 16A).
A B c
Frc. 16. Three stages in the embryonic development of a Segestria. After Holm.
The appearance of two of the germinal layers in this way is followed
by the formation of a temporary coelom, consisting of a series of
metameric sacs. At this stage the division of the developing body into
somites is clear (Fig. l6B).
The appendages begin to be formed at about this stage (Fig. 16C).
The chelicerae appear first, to be followed by the rest in succession, from
before backward. The chelicerae and pedipalpi are at first behind the
mouth, the former assuming their pre-oral condition later. Appendages
of the opisthosoma, though normally absent after hatching, make a
temporary appearance in varying numbers as rudimentary knobs. They
are most numerous in Solifugae, where nine or ten pairs of small
tubercles are to be seen: they are few est in Pseudoscorpiones and
Opiliones (Fig. 17), which show only four pairs.
Frc. I 7. Two stages in the embryonic
5. EMBRYOLOGY: DEVELOPMENT 47
of a harvestman. After Holm.
The course of embryonic development in Solifugae, Uropygi and
spiders, produces an organism which can be described as vnapped round
the central mass of volk with its dorsal surface innermost and concave
and its ventral surf~ce outermost and convex. This position has to be
altered by a process known as inversion or reversion.
At or soon after the time at which the appendages make their
appearance, the embryo, which has the form of a strip of developing
cells, divides longitudinally into two similar halves, united only at the
anterior and posterior ends. The two halves move apart, shortening the
axis of the body, bringing its two ends closer together on the ventral
side with a corresponding lengthening of the dorsal aspect. At the same
time the two halves fold lengthways, enabling their edges to meet and
unite, re-forming the dorsal surface. The whole process of reversion is a
remarkable one, and gives an inevitable impression of a hasty attempt
to repair a mistake that had occurred earlier in the development.
\Vith the completion of the formation of at least the greater part of
the systems of internal organs, the arachnid is ready for hatching. It is
still surrounded bv the inner vitelline membrane and the outer chorion,
and these must b~ split. This splitting is nearly always facilitated by the
use of an egg-tooth, a small hard projection either on the chelicerae or
on the clypeus of the animal. l.:sually it carries an adequate quantity
of yolk, which enables it to survive until it undergoes its first ecdysis and
is, normally, able to feed itself; but most interesting exceptions to this
are found in Scorpiones (Fig. 18) and Pseudoscorpiones.
The eggs of scorpions are yolkless, or nearly so, by the time that the
embryo has completed development to the corresponding to
reversion. The embryos develop each in a separate diverticulum of the
tubular ovary, and the distal end of the diverticulum is drawn out into
an appendix, which lies freely in the haemolymph and absorbs nourishment
from it. The middle portion of the appendix serves as a reservoir,
while the proximal end is a hollow chitinized teat, inserted into the