90 SEX IN MICROORGANISMS 



cell move into the jelly mass between the cells. In Rhopalodia gibba 

 (Klebahn, 1896) the jelly is restricted and appears as a bridge be- 

 tween the two cells. Two zygotes are formed in this bridge. This is 

 believed to be the most primitive type of auxospore formation. 



Isogamous fusion may also take place within a copulation tube. 

 In this type the zygote is formed within the tube. This type of fusion 

 occurs in Eimotia arciis and Eimotia flexnosa (Geitler, 1951a,b). 

 In Navicula radiosa each half of each mother cell rotates through an 

 arc of 90 degrees and the two gametes copulate hi situ (Geitler, 

 1952d). 



Anisogamous fusion occurs if one gamete is active and the other 

 is passive. As in isogamous fusion this may occur with or without a 

 copulation tube. In Gomphoneina parviiluni var. inicropus usually 

 four gametes are involved. One gamete migrates into the other mother 

 cell and fuses with the passive gamete. This stimulates the other 

 gamete to move out and into the first mother cell. As a result two 

 auxospores are formed. Of less frequent occurrence in Gomphoneina 

 pamihnn var. 7/iicropus is the production of only one zygote from 

 two gametes. Of rare occurrence is the production of one zygote 

 from three gametes (Geitler, 1932). 



Of common occurrence in anisogamous fusion is the production 

 of a copulation tube. Depending on the species of diatom, one or two 

 tubes may be produced. 



Usually one tube is produced, as in many species of Nitzschia 

 in which the gametes pass in succession through the tube. If two tubes 

 are present, as in Frnstulia rhoiiiboides \'ar. saxonica, the two fusions 

 may take place at the same time (Geitler, 1949b). 



An unusual type of anisogamous fusion is that reported for 

 Navicula halopbila (Subrahmanyan, 1945), Syiiedra uhia (Geitler, 

 1935), and Syiiedra nmipens var. fragilarioides (Geitler, 1952f). In 

 these species two active gametes are formed in one riiother cell and 

 two passive gametes in the other mother cell. The resulting fusion 

 produces two auxospores in the same mother cell. No copulation tube 

 is formed. 



Automixis is not common in diatoms. Several cases \\'hich need 

 further investigation indicate that this is the means of reproduction. 

 In no Ciise is the nuclear behavior thoroughly understood. What seems 

 to be a true case of autogamy is described for Aviphora norvhwii 



