SEXUAL Rl PRODUCnON IN DIATOMS 87 



rhc two nuclei formed in this division usually degenerates. Sometimes 

 it is reabsorbed in the protoplasm as in Goinphouema parvulmn var, 

 iincropus (Geitler, 1932). In other instances it becomes pycnotic and 

 remains in the gamete as in Ejmotia arcus and Eunotia flexuosa (Geit- 

 ler, 1951a, b) or is cut off as in Aviphipleura pellucida. However, in 

 NiViiciila radiosa (Geitkr, 1952d) and Navicida cryptocephala var. 

 vcneta (Geitler, 1952f) both nuclei remain functional, so that each 

 gamete has two functional nuclei. 



One or two functional gametes may develop in each cell. If two 

 gametes develop, they usually change position. Instead of lying paral- 

 lel to the valves in the position in which they are formed, they come 

 to lie one above the other when viewed from the apex of the cell. 

 In Navicula radiosa (Geitler, 195 2d) this change of position of the 

 gametes does not occur. 



One notable exception to this type of gamete formation is that 

 found in Eunotia arcus and Eunotia flexuosa (Geitler, 1951a,b,c). 

 In these species there is a transverse differentiation of protoplasts. 

 In Eunotia arcus one chloroplast becomes very large, while in Eunotia 

 flexuosa they both move to the same side of the cell. The spindle of 

 the first meiotic division is formed so that one pole is close to the 

 epitheca. Thus a very unequal cell division takes place. The larger 

 cell develops into a gamete. The smaller cell forms what Geitler calls 

 a "remaining cell." Geitler thinks that this remaining cell, by affect- 

 ing the osmotic pressure of the cell, brings about the movement of 

 the gamete. After cytokinesis occurs, the second meiotic division 

 follows in both the gamete and the "remaining cell." One nucleus in 

 each degenerates. 



Another exception is, for example, in Cymbella ventricosa var. 

 (Geitler, 1932) where parthenogenesis occurs. In such cases reduc- 

 tional division does not take place. 



Sometimes it happens, as in Eunotia arcus (Geitler, 1951b), that 

 the cells resulting from meiosis develop shells and become vegetative 

 cells rather than gametes. This phenomenon has been observed several 

 times in diatoms. 



The sex differentiation of gametes, if it occurs, takes place during 

 meiosis. Geitler (1932) thinks that the anisogamy recognized by the 

 difference in size and movement is more apparent than real. He states 

 that movement is due to tensions which develop in the cell. This is 

 well described for Aniphipleura pellucida (Geitler, 1952c). The first 



