32 COMPARATIVE MORPHOLOGY OF FUNGI 



Retardation of development, similar to that in the gametangia 

 before the gametes were individualized, may be recognized in the zygotes 

 and coenozygotes; in them caryogamy is first postponed and then shifted 

 in place. These tendencies will be discussed more in detail in the 

 Ascomycetes. 



The life cycle of most forms resembles that of the simpler Chloro- 

 phyceae: the thallus and the fructification is haploid and the diplont is 

 limited to a zygote at whose germination meiosis probably occurs. 



The systematic classification of the Phycomycetes is based on the 

 developmental forms of the haplont and the structure of the sexual cells. 

 Three orders are generally distinguished, the Chytridiales, the Oomycetes 

 and the Zygomycetes. In the Chytridiales, the thallus is unicellular 

 and poorly developed; in the Oomycetes and Zygomycetes it consists 

 of a highly developed, generally ramose, aseptate, coenocytic mycelium. 

 The Oomycetes are heterogamous; the content of their female gametangia 

 changes into one or more egg cells which are fertilized by sperms or 

 undifferentiated sexual cells (oogamy). The Zygomycetes are isoga- 

 mous; the contents of their gametangia is not further differentiated, but 

 mixes with the content of a second morphologically equivalent gametan- 

 gium (zygogamy). The Chytridiales and Oomycetes are generally 

 aquatic, the Zygomycetes terrestrial. The question of their origin and 

 mutual relationships will be discussed later under the individual orders 



