The walls of the ovary are lined internally by a layer of {132} nucleated cells, separated from the cavity of the organ by a delicate structureless membrane. The growth of these cells gives rise to papillary elevations which project into the cavity of the ovary, and eventually become globular bodies attached by short stalks, and invested by the structureless membrane as a _membrana propria_ (fig. 32, _m_). These are the _ovisacs_. In the mass of cells which becomes the ovisac, one rapidly increases in size and occupies the centre of the ovisac, while the others {133} surround it as a peripheral coat (_ep._). This central cell is the _ovum_. Its nucleus enlarges, and becomes what is called the _germinal vesicle_ (_g.v._). At the same time numerous small corpuscles, flattened externally and convex internally, appear in it and are the _germinal spots_ (_g.s._). The protoplasm of the cell, as it enlarges, becomes granular and opaque, assumes a deep brownish-yellow colour, and is thus converted into the _yelk_ or _vitellus_ (_v._). As the egg grows, a structureless _vitelline membrane_ is formed between the vitellus and the cells which line the ovisac, and incloses the egg, as in a bag. Finally, the ovisac bursts, and the egg, falling into the cavity of the ovary, makes its way down the oviduct, and sooner or later passes out by its aperture. When they leave the oviduct, the ova are invested by a viscous, transparent substance, which attaches them to the swimmerets of the female, and then sets; thus each egg, inclosed in a tough case, is firmly suspended by a stalk, which, on the one side, is continued into the substance of the case, while, on the other, it is fixed to the swimmeret. The swimmerets are kept constantly in motion, so that the eggs are well supplied with aërated water. {134} [Illustration: FIG. 34.—_Astacus fluviatilis._—A–D, different stages in the development of a spermatozoon from a seminal cell; E, a mature spermatozoon seen from the side; F, the same viewed _on face_ (all × 850); G, a diagrammatic vertical section of the same.] The testis consists of an immense number of minute spheroidal vesicles (fig. 33, A, _a_), attached like grapes to the ends of short stalks (_b_), formed by the ultimate ramifications of the vasa deferentia. The vesicles may, in fact, be regarded as dilatations of the ends and sides {135} of the finest branches of the ducts of the testis. The cavity of each vesicle is filled by the large nucleated cells which line its walls (fig. 33, B), and, as the breeding season approaches, these cells multiply by division. Finally, they undergo some very singular changes of form and internal structure (fig. 34, A–D), each becoming converted into a flattened spheroidal body, about 1‐1700th of an inch diameter, provided with a number of slender curved rays, which stand out from its sides (fig. 34, E–G). These are the _spermatozoa_.