CHYTRIDIALES 81 



in Zygochytrium has long been doubted, as has, indeed, the existence 

 of the genus itself. The type of conjugation appears to be no more 

 aberrant, however, than that found in Siphonaria or Polyphagus, ex- 

 cept that the polycentric Zygochytrium is monoecious. According to 

 Sorokin, two short lateral opposing outgrowths are formed, one on 

 each of the main branches of the thallus. The tips of these outgrowths, 

 after fusing, enlarge and, as in a species of Mucor, become delimited 

 as like-sized gametangia (Fig. 4 R, p. 72). The contents of the two 

 gametangia then fuse (Fig. 4 S), and from the resulting zygote is formed 

 a thick-walled rough resting spore (Fig. 4 T), which, upon germination, 

 produces in turn a hypha-like structure. 



In Tetrachytrium fusion of isogamous posteriorly uniflagellate swarm- 

 ers takes place. The biflagellate zygote does not become transformed 

 into a resting structure, but germinates at once to form a new plant. 

 The behavior of the zygote is so remarkable in Tetrachytrium, as com- 

 pared with all other chytrids, that Sorokin's observations should prob- 

 ably here be regarded with a certain degree of skepticism (Fig. 38 



I-M, p. 600). 



Couch (1939b) has presented strong but not conclusive evidence 

 that heterothallism occurs in Pringsheimiella dioica and Rhizophlyctis 

 rosea. In Pringsheimiella, an obligate parasite of Achlya, isolated strains 



each nucleus; spindle is intranuclear. E. Portion of sporangium showing 

 nuclei in process of simultaneous division; at poles of some spindles are 

 kinoplasmic masses with radiating striae. F. Part of sporangium in which 

 zoospores have been delimited; each nucleus bears an oil globule adjacent 

 to it; both globu'e and nucleus are surrounded by dark-staining chromidial 

 material. G. Two conjugating thalli; male nucleus has already passed into 

 enlarged spiny thick-walled tip of conjugation tube, and larger, female 

 nucleus is about to do so. H. Spiny resting spore with the two gamete nuclei 

 lying side by side; smaller, male nucleus soon becomes equal in size to female; 

 later both become smaller, owing to extrusion of chromidia into cytoplasm; 

 deep-stained chromidial material is scattered throughout cytoplasm. /. Devel- 

 oping sporangium, formed at germination of resting spore; the two gamete 

 nuclei have migrated into sporangium and are close together, probably fusing. 

 J. Sporangium formed at germination of resting spore; the two gamete nuclei 

 are now fused. K. Division of large fusion nucleus into two nuclei in sporan- 

 gium formed by germinating resting spore. 

 (Wager, 1913) 



