SAPROLEGNIALES 817 



Petersen (1905) has described the sequence of changes which occur in 

 the protoplasm of the developing thallus as follows : 



1. Stage of ordinary protoplasm: Protoplasm dense with few vacuoles; 

 nuclei in process of division. 



2. Globular stage: Division of nuclei has been achieved as well as differ- 

 entiation of zoospores. Little globular bodies closely packed in sporangium. 



3. Foamy stage: Protoplasm with numerous large vacuoles. Zoospores 

 invisible. Nuclei appear in large numbers in protoplasmic bridges between 

 vacuoles. (Fig. 5 of Lowenthal.) 



4. Stage at which zoospores are regularly disposed against the wall. 

 (Distinguishable from stage 2 only in the older condition of the sporangium 

 and the more regular arrangement of zoospores against the wall.) 



After the foamy stage there occurred a very strong contraction of the 

 plasma, which lasted twenty minutes and during which protoplasmic 

 threads united it to the host wall. Dilation then occurred and the foamy 

 aspect disappeared. Differentiation of the zoospores took place during 

 the period of dilation. When this was half over a certain number of 

 zoospores had already assumed motility, and when dilation was at 

 maximum all the zoospores were in motion. After a period of swarming, 

 the zoospores again became immobile and encysted, forming a layer 

 against the wall. Presumably they eventually escaped from their cysts 

 and emerged through the open discharge tube, leaving behind a reticu- 

 lum (net sporangium) of cyst walls. As has been previously indicated, it 

 is not certain that encystment of the zoospores within the sporangium 

 necessarily precedes emergence. In both open and unopened sporangia 

 the mature zoospores form a thin peripheral layer around a large 

 central vacuole (Fig. 61 M, S, p. 818). Possibly under favorable con- 

 ditions the zoospores may emerge at once upon the deliquescence of the 

 papilla (Fig. 61 O-Q), whereas if external conditions are unfavor- 

 able they encyst and only escape after the return of suitable envi- 

 ronmental circumstances (Fig. 61 R). The earlier observations of 

 Rattray (1887) on the zoospores indicated that they were negatively 

 phototropic. 



The presence of a net sporangium in Eurychasma (Fig. 61 N, p. 818) 

 was believed by Petersen to be of considerable taxonomic import and 

 led him to establish the family Eurychasmaceae. Scherffel (1925a: 4) and 



