The _testis_ (fig. 31, _t_) is somewhat similar in form to the ovary, but, the three divisions are much narrower {130} and more elongated: the hinder median division lies under the heart; the anterior divisions are situated between the heart behind, and the stomach and the liver in front (figs. 5 and 12, _t_). From the point at which the three divisions join, proceed two ducts, which are termed the _vasa deferentia_ (fig. 31, _vd_). These are very narrow, long, and make many coils before they reach the apertures upon the bases of the hindermost pair of walking limbs, by which they open externally (fig. 31, _vd′_, and fig. 35, _vd_). Both the ovary and the testis are very much larger {131} during the breeding season than at other times; the large brownish-yellow eggs become conspicuous in the ovary, and the testis assumes a milk-white colour, at this period. [Illustration: FIG. 32.—_Astacus fluviatilis._—A, a two-thirds grown egg contained in its ovisac (× 50); B, an egg removed from the ovisac (× 10); C, a portion of the wall of an ovisac with the adjacent portion of the contained egg, highly magnified; _ep_, epithelium of ovisac; _gs_, germinal spots; _gv_, germinal vesicle; _m_, membrana propria; _v_, vitellus; _vm_, vitelline membrane; _w_, stalk of ovisac.] [Illustration: FIG. 33.—_Astacus fluviatilis._—A, a lobule of the testis, showing _a_, acini, springing from _b_, the ultimate termination of a duct (× 50). B, spermatic cells; _a_, with an ordinary globular nucleus _n_; _b_, with a spindle-shaped nucleus; _c_, with two similar nuclei; and _d_, with a nucleus undergoing division (× 600).]