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 

 Closterium 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 Rhopalodia 



i. 



2. 



5- 



FIG. 40. 



6. 



Germination of Closterium, after Klebahn. i. Zygote before nuclear fusion. 2. Fir.^t 

 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 (Fig. 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. fahrti., vol. \xi.x. 



