72 FECUNDATION ; NON-MOTILE ISOGAMETES. 



formation of these balls was not determined. The union of the nuclei, 

 which are in the resting stage, does not take place until the germina- 

 tion of the zygote. The behavior of the fusion nucleus, although 

 somewhat beyond the province of our subject, is of such a nature as 



G 



FIG. 21. Fusion of sexual nuclei during germination of the zygote in Closterium. (After Klebahn.) 



A, mature zygote with two large chloroplasts, the two sexual nuclei in contact. 



B, beginning of germination ; the sexual nuclei have fused while in the resting condition. 



C, contents escaping from old wall of zygote ; fusion nucleus in prophase of division. 



D, protoplast free from wall of zygote, fusion nucleus in anaphase of division. 



E, daughter nuclei reconstructed, division of cell begun. 



F, spindle stage of second mitosis ; the nuclei lie on opposite sides of cell near periphery. 



G, second mitosis complete. 



H, cell-division has taken place; in each daughter cell one of the two nuclei is much smaller and 



denser ; the two large nuclei are provided with membranes. 

 I, the daughter cells have begun to assume form of adult cell ; in each the large nucleus which persists 



as the nucleus of the cell, has taken a central position; the smaller one lies near one end of cell. 



to merit attention, especially in connection with the nuclear behavior 



previous to the sexual process in the diatoms to be mentioned below. 



The union of the sexual nuclei in Closterium and Cosmarium, 



