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, T). (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 Spzrogyra by Chmielewskij (see p. 70) : 
(1) 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. 
Fic, 22,—Formation of gametes in Rhopfalodia 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 
