86 SEX IN MICROORGANISMS 



Geitler (1932) thinks that tensions which develop in this jelly as a 

 result of its viscosity determine to some extent the movement of the 

 gametes and the position of the developing zygote and auxospore. 



In many species a large quantity of jelly is produced and the 

 copulating cells are embedded in it. However, in other species the 

 jelly is only represented by the formation of a copulation tube or 

 tubes. In Ewiotia arcus and Ewiotia flexuosa (Geitler, 1951c) the 

 copulation tube is formed by papillae which are formed by each of 

 the two copulating cells. 



The number and shape of the tube or tubes may vary. Usually 

 only one tube is formed; however, in Fnistulia rhoviboides var. 

 saxonica (Geitler, 1949b) two tubes are present. The tube may be 

 long and narrow as in Eimotia arais (Geitler, 1951b) or short and 

 narrow as in Nitzschia subtilis and Amphipleura pellucida (Geitler, 

 1932, 1952c). Usually the tube is formed at or near the middle of 

 the longitudinal axis. In Fnistulia rhomboides var, saxonica (Geitler, 

 1949b) one tube is found near each of the apices of the cell. In Eimo- 

 tia arcus and Ewwtia flexuosa (Geitler, 1951a,b) a tube may be 

 formed at either end of the cell or on the girdle face of the dorsal or 

 ventral side of the valve. 



GAMETOGENESIS 



So far as is known, meiosis occurs by means of two meiotic divi- 

 sions. The spindle lies in the pervalvar axis of the cell. Sometimes it 

 is tipped slightly to one side as in Aviphipleura pellucida (Geitler, 

 1952c). The prophase of the first meiotic division appears to be nor- 

 mal, but, owing to the high numbers of chromosomes and the small 

 size of the nucleus in diatoms, it is difficult to make out all the stages. 

 The two nuclei resulting from the first meiotic division are usually 

 normal. However, in Cocconeis one of the nuclei forms a polar body. 

 It is believed to be an aborted gamete. In Navicula seminulum (Geit- 

 ler, 1932), one nucleus becomes pycnotic and is ejected, whereas in 

 Navicula cryptocephala var. veneta one nucleus is reabsorbed (Geit- 

 ler, 1952f). 



Cytokinesis usually follows the first meiotic division. Previous to 

 this the chloroplasts usually have divided. The cell membrane is de- 

 veloped in a plane parallel to the valves of the cell. 



The second meiotic division usually follows cytokinesis. One of 



