eds, consists
of two distinct parts; one of which is more viscid, and probably more
difficult of digestion, and more nutritive than the other; and this latter
is used in the last week of incubation. The yolk of the egg is a still
stronger or more nutritive fluid, which is drawn up into the bowels of the
chick just at its exclusion from the shell, and serves it for nourishment
for a day or two, till it is able to digest, and has learnt to choose the
harder seeds or grains, which are to afford it sustenance. Nothing
analogous to this yolk is found in the fetus of lactiferous animals, as the
milk is another nutritive fluid ready prepared for the young progeny.

The yolk therefore is not necessary to the spawn of fish, the eggs of
insects, or for the seeds of vegetables; as their embryons have probably
their food presented to them as soon as they are excluded from their
shells, or have extended their roots. Whence it happens that some insects
produce a living progeny in the spring and summer, and eggs in the autumn;
and some vegetables have living roots or buds produced in the place of
seeds, as the polygonum viviparum, and magical onions. See Botanic Garden,
p. 11. art. anthoxanthum.

There seems however to be a reservoir of nutriment prepared for some seeds
besides their cotyledons or seed-leaves, which may be supposed in some
measure analogous to the yolk of the egg. Such are the saccharine juices of
apples, grapes and other fruits, which supply nutrition to the seeds after
they fall on the ground. And such is the milky juice in the centre of the
cocoa-nut, and part of the kernel of it; the same I suppose of all other
monocotyledon seeds, as of the palms, grasses, and lilies.

II. 1. The process of generation is still involved in impenetrable
obscurity, conjectures may nevertheless be formed concerning some of its
circumstances. First, the eggs of fish and frogs are impregnated, after
they leave the body of the female; because they are deposited in a fluid,
and are not therefore covered with a hard shell. It is however remarkable,
that neither frogs nor fish will part with their spawn without the presence
of the male; on which account female carp and gold-fish in small ponds,
where there are no males, frequently die from the distention of their
growing spawn. 2. The eggs of fowls, which are laid without being
impregnated, are seen to contain only the yolk and white, which are
evidently the food or sustenance for the fut