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 cells. 



FIG. aa. Formation of gametes in Rhopalodia gibba. (After 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. 



E, 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 this 

 that our attention is especially directed. 



In Rhopalodia, the form studied by Klebahn, two individuals place 

 themselves side by side, being held together by means of mucilaginous 

 masses. The protoplast of each cell, which contains one nucleus and 



