46 



CRUSTACEA BRANCHIOPODA 



devoured by tlie 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- 

 ecpcr 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 



Fig. 16. — Sketch of a parthenogenetic Moina recti rostris, x 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 tits against the carapace in the natural condition so as 

 to close the brood-pouch. 



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. 



The summer-eggs are always carried until they are hatched \ 

 liy 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 fy-jlj 



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