84 SEX IN MICROORGANISMS 



Iyengar and Subrahmanyan (1944) observed in Cyclotella me- 

 jieghiniana zygote formation and subsequent auxospore formation. 

 They considered this the result of autogamy and automixis. 



Recently von Stosch (1950) has reported oogamy in Melosira 

 varians. He has observed the filaments to be of two types, narrow 

 male filaments and broader female filaments. In about 9 per cent of 

 the cases where the size of the filaments overlap they are found to be 

 monoecious. 



The antheridial cells undergo reduction division. Two sperma- 

 tozoid mother cells bud off the main cytoplasm in the second meta- 

 phase and then divide to form four spermatozoids. Flagella have not 

 been seen, but it is presumed that the spermatozoids are flagellated. 



The young oogonia resemble vegetative cells in shape, yet their 

 plastids and chromatophores are larger. Meiosis takes place in the 

 usual manner. In the first telophase one of the daughter nuclei grad- 

 ually aborts. In the second telophase one of the nuclei becomes pyc- 

 notic. The two "polar bodies" thus formed are gradually absorbed. 



The oogonium swells, and a strip of naked protoplasm is exposed 

 between the margins of the epitheca and hypotheca. The sperma- 

 tozoid may enter the oogonium as early as anaphase I or as late as 

 the maturation of the t^g nucleus. Later the zygote is released and 

 swells to form a subglobose auxospore. 



Geitler (1952e) has recently observed in Cyclotella sp. sexual 

 reproduction similar to that described for Melosira varians. There 

 seems to be no morphological difference in the filaments that form 

 the eggs and sperms. In spermatogenesis there occurs a first meiotic 

 and a second meiotic division which result in the formation of four 

 sperms. The sperm enters the oogonium by the time diakinesis takes 

 place. As a result of a first meiotic and second meiotic division of 

 the nucleus, one ^gg nucleus and two pycnotic nuclei are produced. 

 The sperm nucleus migrates during interkinesis from the peripheral 

 region of the ^gg to the center, where fusion with the tgg nucleus 

 takes place after the second meiotic division is complete. A metagamic 

 mitosis occurs between the formation of the first and second shells of 

 the cell formed in the germination of the auxospore. Although meta- 

 gamic mitosis has been observed many times in the Pennales, this is 

 the first time it has been observed in the Centrales. 



