CHYTRIDIALES 85 



p. 41 1). Each zoospore gives rise to a new thallus. Direct transformation 

 of the resting spore into a zoosporangium rather than a prosporangium 

 has been occasionally described, notably in Rhizophydium messanensis 

 (Morini, 1896), Rhizophydium transversum (Dangeard, 1900-1 901 e), 

 Rhizophydium ovatum (Couch, op. cit.), Diplophlyctis intestina (Zopf, 

 1884), and Cladochytrium replicatum (Karling, 1935). 1 



The so-called "cyst" formed in Polyphagus eug/enae (Dangeard, 

 1900 1901c; Wager, 1913) is perhaps to be regarded as a less special- 

 ized type of resting spore. It is produced simply by the thickening of 

 the wall of the prosporangium and upon germination functions in 

 turn as a typical thin-walled prosporangium. 



Cytology 



While knowledge of the cytology of the aquatic chytrids is increasing, 

 it is still fragmentary. The most complete accounts are those dealing 

 with the interbiotic monocentric Polyphagus eug/enae (Wager, 1899a, 

 1913; Dangeard, 1 900-1901 c), the endobiotic polycentric Cladochy- 

 trium replicatum (Karling, 1937b), and the endobiotic monocentric 

 Endochytrium operculatum (Hillegas, 1940). Since these studies, cy to- 

 logical investigations of certain phases of other types have appeared. 

 In the treatment below the older accounts are given precedence, the 

 more recent are discussed on pages (92-104) that follow. 



Both Wager and Dangeard were in essential agreement as to the 

 nuclear behavior in Polyphagus euglenae, details of the sexual stage 

 of which have been described above (see "Sexual Reproduction," p. 

 75). From Wager's account, the thallus is one-celled and has in the 

 rudiment of the prosporangium a single more or less spherical nucleus 

 (Fig. 5 B, p. 80). In the resting state this nucleus exhibits a central 

 region of lightly stained meshwork connected by delicate radiating 

 threads to the nuclear membrane. A deeply stained arc-shaped cap 

 lying at one side contains all the chromatin material. Surrounding the 

 nucleus is a coarse, densely granular network having a strong affinity 

 for chromatin stains. Deeply staining knots which occur on this retic- 

 ulum also give a chromatin reaction and are considered to have some- 

 thing to do with oil formation. Whether the precise origin of this so- 



1 See, however, Karling, Torreya, 41: 108. 1941. 



