THE BRITISH WOODLICE.
47
parent, it is necessary for the young creatures to be well
supplied with nutritive material. In fact, the bulk of the large
egg is made up of food-yolk, on the outside of which the formative
protoplasm is disposed in irregular patches. In the fertilized
ovum, one of the latter, which lies in a particular position at the
end, is found to be larger than the others (see fig. 22). It
contains the nucleus of the egg-cell (see fig. 23) and is called the
cicatricula. This is the only portion of the egg which divides and 
produces nucleated cells. It is these which gradually 'spread all
over the surface of the food-yolk, forming a layer known as the
blastoderm, which is at first but one cell thick (see figs. 24, 26,
and 28).
Before, however, the food-yolk is quite closed in, a differ-
entiation into two layers—the pro-ectoderm and pro-endoderm—
takes place (see fig. 25) and rudiments of the first two pairs of
appendages appear (see fig. 26). Moreover, the cells of the
ectoderm change their shape and begin to multiply at two
points to form the beginnings of the cerebral ganglia and the
nerve cord respectively.
As the blastoderm closes over the food-yolk, two more
appendages arise and these are soon followed by others (see fig.
28). A depression appears at the point where the blastoderm
closed and internally the pro-endoderm or inner layer is differ-
entiated into two—the endoderm proper and the mesoderm (see fig.
29). The former begins to grow so that its edges unite to form
the middle part of the intestine (see fig. 29) seen from the outside
in fig. 30. The depression already mentioned grows deeper,
forming a tube which is the hind portion of the intestine, while at
the anterior end of the embryo the front part of the intestine is
similarly formed (see fig. 30). By this time also all the nineteen