50 AQUATIC PHYCOMYCETES 



although occasionally, as in Polyphagia euglenae and Obelidium, it 

 may be so. In the majority of genera exhibiting the Rhizidium type 

 of development the thallus is monocentric. It is probable that when 

 the development of such polycentric genera as Physocladia, Nowakow- 

 skiella, Megachytrium, Zygochytriwn, Tetrachytrium, and the like, is 

 more fully understood, they too will be found to be of the Rhizi- 

 dium type. 



In concluding any account of development in the chytrids it should 

 be pointed out that much observational and experimental work still 

 needs to be done before it can be determined just how static the types 

 really are, however distinctive they may appear under natural conditions. 



When the developmental types discussed above were first proposed 

 (Sparrow, 1935a, 1943), Whiffen (1944) took exception to their recog- 

 nition in spite of emphasis given the fact that they are practical arbi- 

 trary groups only and should be so considered. She felt that sufficient 

 was already known to be able to lay the foundation for a natural system 

 based on types of development within the Chytridiales. To this end, 

 she suggested recognition of two groups and five types among the eu- 

 carpic, monocentric chytrids. Group I contained those forms in which 

 the encysted zoospore is functional; Group II, those in which the en- 

 cysted zoospore is not functional and the zoosporangium or prospo- 

 rangium develops from the germ tube. Her scheme is quoted below 



Group I 



Type 1. The encysted zoospore enlarges into a sporangium, as in RJiizo- 

 phydium or Rhizidium. 



Type 2. The encysted zoospore enlarges into a prosporangium from which 

 the zoosporangium develops as in Polyphagus. 



Type 3. A swelling of the germ tube gives rise to a prosporangium 2 

 that contributes to the subsequent enlargement of the encysted 

 zoospore into a zoosporangium as in Chytridium lagenaria (the 

 endo-exogenous type of development described by Karling, 

 1936[a]. 



1 Note that Whiffen's use of the term "prosporangium" includes not only the 

 enlarged body of the zoospore out of which at maturity grows a sporangium, but 

 also the structure (termed herein an "apophysis" or "subsporangial swelling"), 

 which forms at the tip of the germ tube of the zoospore within the substratum, 



