Life-cycle of the Lower Crustaceans 345 



development of the parthenogenetic and the "winter" egg that 

 have a bearing on the present question. The wall of the ovary 

 swells up, and its cells become filled with a fluid when the par- 

 thenogenetic egg is about to develop. The egg seems to get its 

 nourishment from these cells. On the other hand, when the 

 winter egg develops the walls of the ovary do not enlarge, but the 

 egg develops at the expense of the surrounding cells that afford 

 the nourishment for its growth. The conditions in the ovary 

 itself would seem, therefore, to be determinative as to which 

 kind of egg will develop. It should be observed, however, that 

 we do not know whether it is the same cell that becomes in one 

 case a winter egg and in the other a parthenogenetic egg. In 

 fact, Weismann has stated that the two kinds of eggs come from 

 different parts of the ovary. The special conditions that lead 

 to formation of the male egg are unknown. 



LITERATURE, CHAPTER XXI 



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 ISSAKOWITSCH, A. Geschlechtsbestimmende Ursachen bei den Daphni- 



den. Biolog. Centralbl. XXV. 1905. 

 KERHERVE, L. B. DE. De 1'apparition prevoquee des males chez les Daphi- 



nies (Daphnia psittacea). Mem. soc. zool. France, VIII. 1895. 

 KURZ, WILH. Dodekas neuer Cladoceren nebst einer kurzen uebersicht 



der Cladocerenfauna Bohmens. S. B. math, naturw. Wien. 1875. 

 SCHMANKEWITSCH, W. J. Zur Kenntniss des Einflusses der aeusseren 



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Zool. XXIX. 1877. 



SIEBOLD, C.V. Sitzungsber.d. Kgl. Akad. d.Wissen. Miinchen,III. 1873. 

 Verhandl. d. 59, Jahresversammlung der Schweiz naturf. Gesell. in 



Basal. 1877. 

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Environment. Quart. Jour. Micro. Sci. XLIII. 1900. 

 WEISMANN, A., and ISCHIKAWA. Ueber die Paracopulation im Daphnidenei 



Zool. Jahrbucher. 1889. 

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Z. wiss. Zool. XXVII. 1876. Theil II-V, Ibid. XXVIII. 1877; 



Ibid. XXX. Supplement (1878); Theil VI-VII, Ibid. XXXIII 



(1879). Separate, Leipzig. 1876-1879. 



