Fig. [15].—A, Ovary of a parthenogenetic Leptodora hyalina; B, base of another ovary of the same species, showing a so-called “nutrient chamber”; C, ovary of a female Daphnia, showing the formation of a winter-egg. E, E1–E3, Parthenogenetic egg; Ep, ovarian epithelium; G, germarium; N.C, nutrient chamber; O.D, oviduct; W, winter-egg; 1, 2, 4, the other three cells of the same group; II, III, two other groups of germ-cells.
The production of a winter-egg is a more complicated process. The epithelium of the ovarian tube swells up, so that the lumen is nearly obliterated, and several sets of four germ-cells pass from the germarium to lie among the swollen epithelial cells. All these groups of germ-cells, except one, disintegrate and are devoured by the ovarian epithelium, one cell of the remaining group enlarging to form a winter-egg, fed during its growth not only by the three cells of its own set but also by the epithelial cells of the ovarian tube, which have devoured the germ-cells of other sets. An ovary never contains more than a single winter-egg at the same time, the number of germ-cells which are devoured during its formation varying in the different species; the Daphnia drawn in Fig. [15], C, has produced three groups of germ-cells, of which two (II, III), will die, while the cell W from the remaining group will develop into an ovum; in Moina, Weismann finds that as many as a dozen cell-groups may be thrown into the ovary before the production of a winter-egg, so that only one out of forty-eight germ-cells survives as an ovum.
Fig. [16].—Sketch of a parthenogenetic Moina rectirostris, × 45, the brood-pouch being emptied and the side of the carapace removed, showing the dome of thickened epithelium on the thorax, by which nutrient material is thrown into the brood-pouch, and the ridge which fits against the carapace in the natural condition so as to close the brood-pouch.
Fig. [17].—Moina rectirostris, ♀, × 40, showing the ephippial thickening of the carapace which precedes the laying of a winter-egg.
The summer-eggs are always carried until they are hatched by the parthenogenetic female which produces them. The brood-pouch is the space between the dorsal wall of the thorax and the carapace. This space is always more or less perfectly closed at the sides by the pressure of the carapace against the body, and behind by vascular processes from the abdominal segments (Figs. [10], 16, etc.). The presence of a large blood-sinus beneath the dorsal wall of the thorax and in the middle line of the carapace suggests the possibility that some special nutrient substances may pass from the body of the parent into the brood-chamber, and in some species the thoracic ectoderm is specially modified as a placenta. In Moina (Fig. [16]) the dorsal wall of the thorax is produced into a dome, covered by a columnar ectoderm, which contains a dilatation of the dorsal blood-sinus; and in this form it has been shown that the fluid in the brood-pouch contains dissolved proteids. Associated with the apparatus for supplying the brood-pouch with nutriment is a special apparatus for closing it, in the form of a raised ridge, which projects from the back and sides of the thorax and fits into a groove of the carapace.
A somewhat similar nutrient apparatus exists in the Polyphemidae, where the edges of the small carapace are fused with the thorax, so that the brood-pouch is completely closed, and the young can only escape when the parent casts her cuticle. In some genera of this family (e.g. Evadne) the young remain in the parental brood-pouch until they are themselves mature, so that when they are set free they may already bear parthenogenetic embryos in their own brood-pouches.
Fig. [18].—Newly-cast ephippium of Daphnia, containing two winter-eggs.