The openings are very difficult to identify, and Lereboullet (39, p. 113) was unable to find them. It is obvious that the openings must be underneath the plates that form the egg pouch, and as a change of skin is required to set these free, it would appear that at ordinary seasons the ducts from the ovaries are closed.
The writers have been able to determine from external examination of specimens which had moulted and were about to lay eggs, that the oviducts at such time open to the inside of the base of each walking leg on the fifth segment. In similar specimens the oviducts were also followed to the opening from within. The brood pouch has already been described.
The male organs consist of six testes arranged in two pairs, each of which is provided with a reservoir (see fig. 21). The efferent ducts from the two reservoirs unite at the base of the thorax to form a common duct (or "penis").
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.
