104 AQUATIC PHYCOMYCETES 



metaphasc the chromosomes usually form a dense equatorial band, 

 which sometimes appears to be composed of several discrete bodies 

 (Fig. 9 J, N). Polar views of the equatorial plate apparently reveal the 

 presence of five or six chromosomes (Fig. 9 J, O); their exact number 

 was impossible to determine. 



During mitosis the nucleolus as indicated earlier is usually lens- 

 shaped, lies close to the nuclear membrane, and persists throughout 

 metaphase and anaphase (Fig. 9 I, J, N, P). Nucleus and spindle elon- 

 gate during anaphase and at telophase the nuclear membrane is no 

 longer clear as a distinct structure. 



Cleavage furrows separating the developing zoospores progress cen- 

 tripetally from the periphery and by their extension and branching 

 eventually cleave out the uninucleate zoospores (Fig. 9, H, Q-X). 

 After zoospore discharge sporangial proliferation is common, the 

 new sporangium arising from an ingrowing of protoplasm with a 

 single nucleus from the concomitant vegetative system. 



Parasitism 



Before their true nature was understood chytridiaceous fungi were 

 often taken by early investigators to be the reproductive structures 

 of the aquatic organisms on which they were living. For example, 

 references are found in the early literature to the formation of zoo- 

 spores by desmids (Archer, 1860), of antherozoids by saprolegnians 

 (Pringsheim, 1860), and of sperms in its eggs by the animal Nais 

 (Carter, 1858). Similar misinterpretations might be cited regarding 

 marine organisms (Wright, 1879b). As knowledge of Braun's mono- 

 graph (1856a) became more general, however, the extraneous nature 

 of the chytridiaceous interloper was recognized. 



Early observers of these fungi generally assumed that when a chyt- 

 rid was found on dead or dying algal cells it was the primary causal 

 agent. Rosen (1887) soon pointed out, however, in connection with 

 a study of Phlyctochytrium zygnematis, that the zoospores of this 

 fungus most often came to rest on obviously dead cells of Zygnema 

 or on green filaments which, because of unfavorable environmental 

 conditions, were fast becoming moribund. This saprophytic tendency 

 of chytridiaceous fungi was later emphasized by Serbinow (1907), 



