CHYTRIDIALES 71 



duces an endobiotic rhizoidal system. The epibiotic part of one thallus 

 now enlarges rapidly and continues to extend its rhizoidal system, but 

 the other apparently ceases to grow. Within the larger thallus at 

 the point of contact can be observed a short refractive tube seemingly 

 formed by the male gametangium (Fig. 4 H). The entire contents of 

 this gametangium, with the exception of a small globule, is then dis- 

 charged through the tube into the larger plant, which continues to 

 grow and is ultimately transformed into a thick-walled resting spore 

 to which the empty cyst of the male gametangium, generally torn 

 from its rhizoidal system, remains adherent (Fig. 4 I). In R. ova turn 

 (Couch, 1935a) and at times in R. granulospomm (Scherffel, 1925b; 

 Sparrow, 1 939a, Fig. 4 J-O), the swarmer which will then give rise to 

 the male gametangium comes to rest on the algal cell and forms a 

 rudimentary rhizoidal system. A second, like-sized swarmer then at- 

 taches itself to the upper part of this gametangium (Fig. 4 B). The 

 second spore, however, never makes contact with the host and never 

 forms a vegetative system. Eventually it receives the contents of the 

 other gametangium and enlarges to form the resting body. In R. ovatum, 

 in which the sequence of development has been carefully followed, 

 both gametangia enlarge at the expense of the host, materials being 

 obtained by the rhizoidal system of the epibiotic male for both it and 

 the more distal receptive thallus. The latter enlarges at a much greater 

 rate and eventually receives the contents of the male thallus through 

 a broad opening formed in the walls at the place of contact. It then 

 becomes converted into a thick-walled smooth resting spore (Fig. 4 

 C-F). A cytological examination of this process shows the gametangia 

 to be uninucleate and reveals that with the merging of the gametes 

 the nuclei as well as the cytoplasm fuse. A thickened wall is then laid 

 down around the zygote, which is supported at the apex of the thin- 

 walled male gametangium. A variation of this process is seen in R. 

 anomalum (Canter, 1950a), in which the contributing thallus develops 

 until it resembles an early stage in sporangial growth. A zoospore 

 then attaches itself to the base of the male thallus and eventually re- 

 ceives the contents of the latter. 



It is clear that in certain species of the large genus Rhizophydium a 

 sexual act precedes the formation of the resting spore. Many phases 



