44 CRUSTACEA— BRANCHIOPODA chap. 



each side of tlie bodj, 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," containing 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 Ltytodora 

 (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^, Eg. 

 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, KG). 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 lie among the swollen epithelial cells. All 

 these groups of germ-cells, except one, disintegrate and are 



^ Zcitsclir. %oiss. Zool. xxiv., 1874, p. 1. 



