Io THE BRIIISH WOODLICE. 
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 
cicatvicula. 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 pvo-ectoderm and pro-endoderm— 
takes place (see fig. 25) and rudiments of the first two pairs of 
FIG. 22.—THE FERTILIZED EGG FIG, 23.—THE FERTILIZED EGG SEEN IN SECTION 
(Porcellio scaber), AFTER ROULE. (Porcellio scaber), AFTER ROULE, 
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 difter- 
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 
