=Development.=--The eggs, in the common species of woodlice, at least, are laid at the beginning of summer, and are retained in the brood pouch, where they undergo their development. The process has been recently traced with great care by Professor Louis Roule (58) in _Porcellio scaber_ and the description which follows is based upon his researches. As, practically speaking, the larval stages are passed within the egg, and there is no free embryo differing in form from the 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 differentiation 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. [Illustration: FIG. 22.--THE FERTILIZED EGG (_Porcellio scaber_), AFTER ROULE.] [Illustration: FIG. 23.--THE FERTILIZED EGG SEEN IN SECTION (_Porcellio scaber_), AFTER ROULE.] 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 differentiated 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 appendages have made their appearance and the mesoderm, (which has grown considerably, to form the beginnings of the muscles) has sent prolongations into each of them. About this time, spaces (see fig. 31) are formed in the muscular mesoderm which are all that remain of the _true body cavity_ characteristic of animals above the level of the jelly fish, and in these spaces the blood ultimately circulates. _SURFACE VIEWS._ [Illustration: FIG. 24.] [Illustration: FIG. 26.]