144
THE ESSEX NATURALIST.
mother. Into this brood pouch the fertilised eggs are at the
same time liberated from the ovary to continue their develop-
ment, external to the actual body cavity of the mother, yet still
enclosed and protected within this brood pouch from the outside
world.
Prior to the moult these brood-plates have apparently been
developing unseen, beneath the hypodermis of the ventral
surface of the mother, the developmental stimulus having
been supplied in some manner by the presence of maturing eggs
within the ovary. These brood-plates, it may be noted, are
completely absent during non-breeding periods, and are therefore
to be regarded as temporary organs whose sole purpose is to
support and protect the newly-laid eggs whilst they undergo
further development ; they disappear at the moult which
eventually succeeds the liberation of the brood, and are
re-constituted at each laying.
The eggs are uniformly granular, spherical, yellowish objects
less than the size of a pin's head, though they vary considerably
in size and also in number according to the species. There is,
however, a fair constancy among the individuals of each species,
sufficient in fact to justify the calculation of average values for
each, as shown in the table below. Moreover, there is also an
interesting relation between the size (i.e., yolkiness) of the eggs
and the number laid, the proportion of one to the other being
in inverse ratio, up to a point at least where the amount of
yolkiness in the egg reaches the minimum necessary for develop-
ment. Thus in T. pusillus, for example, the eggs are exceptionally
large for such a small species, but only five or six constitute a
normal brood : in O. asellus, on the other hand, the eggs are
comparatively small, but an average of thirty are laid at a time,
whilst in A. vulgare, again a species with tiny eggs, as many as
one hundred and thirteen constitutes an average brood.
A further interesting point is that the durations of the
successive stages in the development of these eggs, whilst varying
considerably with the different species, show quite a marked
constancy amongst the individuals of each particular species,
so that it has been possible to compile a table of the average
durations of these developmental periods in the different species,
based upon the records from not less than twenty of each species.