282 Journal of Agricultural Research voi. iv, no. 4 



200 were quite frequent. In the majority of cases the enveloping mem- 

 brane seemed to burst and then contract so as to allow the zoospores to 

 escape almost simultaneously in all directions. It is usually impossible 

 to discover any trace of the membrane after the spores have escaped. 

 During the progress of cleavage the wall seems to function as a semi- 

 permeable membrane, since the distance between it and the cytoplasm 

 increases somewhat and the size of the entire body increases by a few 

 microns. The time which elapses between the egress from the prespo- 

 rangium and the liberation of the zoospores is usually about 30 minutes, 

 though in some instances observed the entire process occupied only 17 

 minutes. 



Under favorable conditions of observation at high magnification one 

 may occasionally note an additional phenomenon, which appears to be 

 related to the first appearance of cleavage furrows at the periphery of 

 the contents of the sporangium. About the time the j&rst indications of 

 cilia are obser\'able small, bubble-like protuberances suddenly pufif out 

 here and there and then instantly disappear as if they had burst. There- 

 after cleavage indentations appear at the points of rupture, which con- 

 tinue to deepen until cleavage has been completed. 



In a few instances the presporangium was observed to develop within 

 the tissues of the host or in submerged agar in such a way as to permit 

 of its reenforcement by the surrounding substance to such an extent that 

 it was not ruptured, but remained intact. In such cases cleavage occurred 

 within the presporangium, and the spores either escaped from the tip of 

 the hypha for a swarm period or remained imprisoned within it, where 

 they germinated. Instances were also observed in which germination 

 occurred within the sporangium without liberation. Such cases, how- 

 ever, were seen only rarely and are not to be regarded as typical. 



Following their escape, the zoospores swim about actively for a time, 

 then come to rest, round up, increase in size to a diameter of about 11 

 or i2/i while developing a large central vacuole, and send out germ 

 tubes, generally two, which develop into typical vegetative mycelium 

 from which zoospores are again produced. In the motile stage the 

 zoospores are plano-convex, with a rather deep sinus on the flattened 

 side. They always possess a single somewhat conspicuous vacuole and 

 are biciliate. They have an average length of 12// and an average 

 width of 7.5//. 



Coincident with the later stages of zoospore production and following 

 it, oospores are formed. The oogonium develops terminally as a spherical 

 body from 22 to 27// in diameter. The antheridium, which develops 

 either terminally or more generally in an intercalary position, is appressed 

 to the oogonium. It is suborbicular, becoming cylindric to broadly 

 clavate, averaging from 9 to 11^ in width by 10 to 14/1 in length. The 

 oospores are spherical and when mature have either a smooth or some- 



