CHYTRIDIALES 75 



(1936d). More recently, Haskins (1950) has shown that an essentially 

 similar process precedes resting-spore formation in D. sexualis. 



Of considerable interest too is the discovery of sexuality by Canter 

 (1951) in the little-known genus Rhizosiphon. Here, an isogamous fu- 

 sion takes place in which the gametes make contact directly, or later- 

 ally by means of a short tube. One gamete only has contact with the 

 host and that by means of a single rhizoid. Although the process as 

 recorded is based upon scanty observations, from the figures and de- 

 scription it appears that one gamete first produces an unbranched germ 

 tube, part of which is extramatrical, and is then joined with the other 

 gamete. The bodies of both remain distinct. The resting spore is en- 

 dobiotic and from its size must have undergone a period of vegetative 

 growth within the host cell. 



Sexuality has also been observed in the Rhizidiaceae. Here, too, the 

 gametic material fuses in elements of one or the other gametangium 

 and is never set free in the medium. Furthermore, in all well-authen- 

 ticated occurrences the gametangia themselves were more or less mature 

 thalli, often of markedly different size. 



The sexual reproduction in the rhizidiaceous genus Polyphagus is 

 perhaps the best-known example of this process among the chytrids. 

 The classic investigations of Nowakowski (1876b) on Polyphagus show- 

 ed that one thallus put out a tube which made contact with another 

 (Fig. 4 Z, p. 72). l In the distal part of this tube the contents (gametes) 

 of both thalli accumulated. The zygote, subterminal in the tube, then 

 became invested with a thick wall and underwent a period of rest 

 (Fig. 4 Z'). At germination it produced a zoosporangium. Although 

 the tube-producing thallus in P. euglenae is frequently smaller than the 

 other one and is ordinarily referred to as the male, in many individuals 

 there is little or no difference between the two. Indeed, in some instances 

 the male may be the larger. Cytological investigations of this sexual 

 process (Wager, 1899a, 1913; Dangeard, 1900- 1901c) show that the 

 two gametes are uninucleate. Wager observed that the male nucleus 

 is the smaller and that it, with the cytoplasm, passes first into the swol- 

 len tip of the tube (Fig. 5 G, p. 80). The female gamete thereupon enters 

 and the two nuclei make contact but do not fuse (Fig. 5 H). The smaller 

 enlarges until it equals the female nucleus in size, and the two move 



1 Illustrations are of P. laevis. 



