. Fecundation in plants. Plant embryology; Plants, Sex in; Karyokinesis. CLOSTERIUM AND COSMARIUM.—DIATOMS. 73 according to Klebahn, occurs just prior to the escape of the contents of the zygote from the outer membrane (Fig. 21, A, B), During the latter process the fusion nucleus often shows signs of approaching karyokinesis (Fig. 21, C). There now follow two karyokinetic divisions in rapid succession, so that each daughter cell may contain two nuclei (for a cell-division may also have taken place) one of which remains as the nucleus of the daughter cell, while the other gradually undergoes di
. Fecundation in plants. Plant embryology; Plants, Sex in; Karyokinesis. CLOSTERIUM AND COSMARIUM.—DIATOMS. 73 according to Klebahn, occurs just prior to the escape of the contents of the zygote from the outer membrane (Fig. 21, A, B), During the latter process the fusion nucleus often shows signs of approaching karyokinesis (Fig. 21, C). There now follow two karyokinetic divisions in rapid succession, so that each daughter cell may contain two nuclei (for a cell-division may also have taken place) one of which remains as the nucleus of the daughter cell, while the other gradually undergoes disorganization (Fig. 21, D, E, F, G, H, I). (See expla- nation of figure for details.) It will now be seen that the process in the zygote of the desmids differs from that described for Spirogyra by Chmielewskij (see p. 70) : (i) in the fusion of the sexual nuclei in the resting stage ; (2) in that there is no second fusion of two of the four daughter nuclei, but a cell-division, one nucleus going to each of the daughter B C Fig. 22.—Formation of gametes in RhopalndiagMa.—{AAa! Klebahn.) A, protoplast of cell showing first mitosis ; nucleus in spindle stage. B, second mitosis, each daughter nucleus dividing, C, second mitosis complete, the four nuclei about equal in size. D, part of two conjugating individuals ; the protoplast of the one on left has begun to divide by becom- ing constricted in the middle; two nuclei in each cell are large, other two have become smaller. £, cell-division complete. DIATOMS. In the diatoms the type of isogamous fecundation resulting in the formation of the auxospore recalls the nuclear history subsequent to fecundation in the desmids. As in the case of the desmids we are indebted also to the investigations of Klebahn ('96) and to those of Karsten (1900), for a more accurate knowledge of the nuclear behavior preceding the sexual act. The nuclear activity, which immediately precedes conjugation, is of prime importance here, and it is to
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