92 SEX IN MICROORGANISMS 



During auxospore development one pair fuses and the other pair de- 

 generates. 



The zygote may be found in various positions. As a result of 

 isogamy it is formed betu^een the mother cells. The polar axis of the 

 zygote is at right angles to that of the mother cells. As a result of 

 anisogamous fusion the zygote is first formed in the mother cell. 

 When two zygotes are formed, one is usually produced in each 

 mother cell. However, in Naviciila halophila (Subrahmanyan, 1945), 

 Synedra ulna (Geitler, 1939), and Syiiedra nimpens var. fragilarioides 

 (Geitler, 1952f) the two zygotes are produced in the same mother 

 cell. Likewise in automixis and parthenogenesis the zygote is first 

 formed in the mother cells. Later it migrates out of the mother cell. 

 Usually in anisogamous fusion the long axis of the auxospore is paral- 

 lel to that of the mother cell, whereas in isogamous reproduction the 

 long axis of the auxospore is perpendicular to the long axis of the 

 mother cells. However, if the jelly surrounding the copulating cells 

 is relatively thin, the auxospores may vary somewhat in position. 

 Geitler (1932) believes that this interesting correlation of the position 

 of the auxospore with type of reproduction is a result of tensions 

 developed within the jelly rather than a result of the type of gametes. 



The zygote elongates in the formation of the auxospore. In this 

 process the zygote membrane often breaks and appears as caps on the 

 ends of the auxospore, as in Frustulia rhomboides var. saxonica (Geit- 

 ler, 1949b). In Anomoeoneis exilis the zygote membrane persists as 

 laminations over the poles of the auxospore (Geitler, 1949b). In 

 Nitzschia fonticola (Geitler, 1932) the zygote membrane is elastic 

 and does not break. 



The perizonium, which is the auxospore membrane, develops un- 

 der the membrane of the zygote. It becomes weakly silicified. The 

 silicification starts at the center of the auxospore and develops out 

 toward the poles. The perizonium may develop a distinctive pattern of 

 markings or be smooth. 



When the auxospore is mature, a nuclear division (metagamic 

 division) occurs. One of the resulting nuclei is pycnotic. This phe- 

 nomenon has been observed in various genera of the Pennales. 



After a period of time the auxospore develops the shells typical 

 of the vegetative cell. The first shell to develop is the epitheca. It is 

 irregular in that it does not have a girdle band. Therefore the edges 

 of the valve bend over and the valve has a curved appearance. The 



