DIATOMACE^E 195 



emerge from the valves and unite to form one body, 

 which then grows into an auxospore. There seems 

 to be some doubt whether there is here a true con- 

 jugation whether these two cells are true motionless 

 gametes and the product a zygote. (4) This process 

 may be gone through, but without union. The two 

 cells give up their contents, which lie side by side 

 unclothed with a membrane, and either close together 

 or separated by a layer of the gelatinous envelope. 

 Each develops independently into an auxospore. 

 (5) Two auxospores may be formed by a pair of 

 Diatoms, which on emerging from the old valves 

 divide each into two. Each pair of the four cells 

 thus formed conjugate and form an auxospore. Re- 

 garding all these processes, it appears to be fair to 

 assume that conjugation, or rather union, whether 

 between one pair or two pairs, is the normal process 

 of forming an auxospore, and that the other cases 

 are parthenogenetic, where a pair produce auxospores 

 together without conjugation, or where one produces 

 two auxospores by itself, or where one similarly 

 produces a single auxospore. The simplest form of 

 all is common ; so is the case of two Diatoms forming 

 their auxospores without conjugation, and of two 

 combining to form one by conjugation, though not 

 so frequent as the others. The case of one Diatom 

 forming two auxospores is known in only one form, 

 viz. lihabdoncma arcuatum ; and the last case, where 

 two Diatoms divide and the daughter-cells conjugate, 

 each pair giving rise to an auxospore, is recorded for 

 Spithemia Zebra only. The mode of formation of 



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