44 CRUSTACEA—BRANCHIOPODA CHAP. 
each side of the body, each continuous with a duct which opens 
to the exterior behind the last thoracic limb. In the female, 
the opening is dorsal (Fig. 10), in the male it is ventral 
(Fig. 11). The external opening is usually simple; but in the 
male there is sometimes a penis-like process, on which the vas 
deferens opens (Daphnella). 
The eggs are of two kinds, the so-called “ summer-eggs,” with 
relatively little yolk, which develop rapidly without fertilisation, 
and the so-called “winter-eggs,” containmg much yolk, which 
require to be fertilised and then develop slowly. 
At one end of the ovary, generally that nearest to the 
oviduct, there is a mass of protoplasm, containing nuclei which 
actively divide; this is the germarium (Fig. 15, A, B,C). As 
a result of proliferation in the germarium, nucleated masses are 
thrown off into the cavity of the ovary; each such mass con- 
tains four nuclei, and its protoplasm soon becomes divided into 
four portions, one round each nucleus, so that four cells are 
produced. In the simpler ovaries, such as that of Leptodora 
(Fig. 15, A), these sets of four cells are arranged in a linear 
series within the tube of ovarian epithelium; in other cases, as 
in Daphnia, the arrangement is more irregular. In the normal 
development of parthenogenetic eggs, one cell out of each set of 
four becomes an ovum, the other three feeding it with yolk and 
then dying. Weismann! has shown that the ovum is always 
formed from the third cell of each set, counting from the 
germarial end, so that in the ovary of Leptodora drawn in Fig. 
15, A, the ova will be formed from the cells marked E, E,, E,. 
At certain times, one or two sets of germinal cells fail to produce 
ova; the epithelial wall of the ovary thickens round these cells, 
so that they become incompletely separated from the rest in a 
so-called “nutrient chamber” (Fig. 15, B, N.C). Germ-cells 
enclosed in a nutrient chamber degenerate and are ultimately 
devoured by the ovarian epithelium. The significance of these 
nutrient chambers is unknown. 
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 le among the swollen epithelial cells. All 
these groups of germ-cells, except one, disintegrate and are 
1 Zeitschr. wiss. Zool, xxiv., 1874, p. 1. 
