NUCLEAR CYCLE IN DESMIDS AND DIATOMS 71 
the fusion-nucleus soon follows (Fig. 40. 2, 3): this is succeeded by 
further mitosis (Fig. 40. 4), with the result that four nuclei are formed: 
two of these are larger than the other two, and a pair of nuclei of unequal 
size, one small the other large, finds its place in each of the two cells 
into which the zygote divides (Fig. 40. 5, 6). Of these nuclei the 
smaller one in each cell disappears, and it is thought that it becomes 
disorganised, while the other remains as the definitive nucleus of the new 
Clostertum cell. 
Certain Diatoms show on conjugation a singular parallelism of behaviour 
to this, but with important differences, and again it is to Klebahn that 
we owe the description of the details.1 In the conjugation of R/opalodia 
Fic. 40. 
Germination of Closterium, after Klebahn. 1. Zygote before nuclear fusion. 2. First 
‘mitosis. 3. Bi-nuclear stage. 4. Second mitosis 5.’ Bi-cellular stage with large and 
small nuclei. 6. Formation of the two germs. (From Oltmanns.) 
(Fig. 41), the protoplasts of the two cells come into close relation, but 
the nuclei remain distinct (Fig. 41. 2), and undergo each a division into 
two, and again into four (Fig. 41. 3, 4): of these, two in each cell are 
soon reduced in size, while the others are distinguished by their larger 
size (Fig. 41. 4). Then comes an abstriction of each of the original 
protoplasts into halves, and each half contains two nuclei, one large and 
one small (Fig. 41. 5): these halves coalesce in pairs, and each fusion-pair 
finally develops into an auxospore (Tig. 41. 6, 7): the two larger nuclei 
of each pair meanwhile fuse, while the smaller nuclei become disorganised 
(Fig. 41. 8). 
In these cases there is conjugation, and a tetrad-division of nuclei 
accompanies it. There is no evidence as yet of chromosome-number, 
1 Pringsh. Jahrb., vol. xxix. 
