PHYCOMYCETES 35 



increases in size at the expense of the host protoplasm and during 

 the period of vegetative growth is termed the thallus. At 

 maturity it possesses a membrane and functions as a sporangium, 

 freeing swarmspores which pass to the outside of the host through 

 a more or less elongate exit tube. When two or more swarm- 

 spores enter the same host cell the resultant thalli sometimes 

 copulate. Though the details of the sexual process in such 

 cases are not well known and may differ in different species, there 

 always exists in later stages a larger thallus united with one or 

 more smaller thalli by open pore connections. The whole content 

 of the smaller cell or cells enters the larger cell, and it then 

 assumes a thick wall and matures into the resting spore. Pre- 

 sumably, therefore, the larger cell is female and the smaller cells 

 are male. At least in some species {e.g., Olpidiopsis vexans) it 

 is known (Barrett 1912) that both the male and female cells have 

 become plurinucleate, through division of the primary swarm- 

 spore nuclei, before the male cells have discharged their contents 

 into the female. Whether at the moment of their initial contact 

 the copulating cells are already provided with thin membranes 

 is not certainly known, but seems likely. If they were then still 

 naked protoplasts their complete merging to form a fusion cell, 

 as in Monochytrium, would probably occur. The very early 

 stages in the process are unknown. Though the copulating 

 cells differ in size from an early period, it is possible that at the 

 beginning of their association they are morphologically equiv- 

 alent. When the male nuclei enter the female cell they appear 

 to fuse in pairs with the female nuclei. The female cell finally 

 assumes a thick wall, and is transformed directly into the resting 

 spore. The use of the term oospore or zygospore in such cases 

 results only in indefiniteness. The .sexual process is clearly 

 neither typically oomycetous nor typically zygomycetous. In 

 the Ancylistales other intermediate types occur, and, though 

 that group as a whole is placed by some students in the Oomy- 

 cetes, zygomycetous tendencies in some of the genera at least 

 {e.g., Ancylistes) are seen in the absence of the oosphere and the 

 direct copulation of the gametangia. In this order copulation 

 is effected in some cases by means of a conjugation tube. In 

 Zygorhizidium willei and Polyphagus euglenae of the Chytridiales 

 uninucleate thalli copulate in similar fashion by means of a 

 conjugation tube. The individual putting out the tube is usually 

 considered to be from that fact alone clearly the male. It is of 



