Phylum Opisthokonta [115 



proximal part of the system of rhizoids develops a large swelling called the apo- 

 physis. In other forms, the center generates the sporangium as an outgrowth. In 

 these circumstances, the center is sometimes called an apophysis, but were better 

 called a presporangium. The sporangium discharges its spores, usually, through one 

 or more tubes which grow forth from it. The tube may open through a difTerentiated 

 cap, the operculum; the production of opercula appears to mark a natural subordi- 

 nate group. 



Syngamy occurs in different chytrids in most of the possible fashions, by union of 

 like or unlike swimming cells, by the union of a swimming cell with a stationary one, 

 or by the establishment of contact by growth. The zygote regularly becomes a thick- 

 walled resting spore (asexual resting spores are also of frequent occurrence). Resting 

 spores germinate by producing zoospores. Meiosis has not been observed, but is be- 

 lieved to occur during the first nuclear divisions in the germinating zygote; the life 

 cycle is apparently of the primitive type, in which all cells except the zygote are 

 haploid {Phy so derma, or at least some of its species, is believed to be exceptional). 

 Sparrow (1943) recognized nine families. One of these does not appear tenable; 

 the remainder are distinguished as follows: 

 1. Sporangia not opening through opercula. 

 2. Eucarpic, i. e., producing rhizoids and 

 sometimes other filaments, the centers 

 not constituting the entire body. 



3. Pluricentric Family 1. Cladochytriacea. 



3. Monocentric. 



4. Germinating spores generat- 

 ing the center as a distinct 



body Family 2. Phlyctidiacea. 



4. Zoospores themselves becom- 

 ing centers, and subsequently 



sporangia or presporangia Family 3. Rhizidiacea. 



2. Holocarpic, i. e., without rhizoids, the 

 individual consisting entirely of one or 

 more centers. 



3. Centers becoming presporangia, 

 each one generating a cluster of 



sporangia Family 4. Synchytriacea. 



3. Centers proliferating, giving rise to 



linear series of sporangia Family 5. Achlyogetonacea. 



3. Each center becoming one spor- 

 angium Family 6. Olpidiacea. 



individual with light sporangia and dark resting cells with pitted walls; h, branch of 

 sexual individual, the oogonia larger and darker than the antheridia; i, gametes. 

 j-m, Allomyces Arbuscula after Hatch (1935); j, k, gametes, x 1,000; 1, m, mitotic 

 figures in the gametangia, x 2,000. n-r, Blastocladiella cystogena, x 500, after Couch 

 and WhifFen (1942); n, individual producing a resting spore; O, resting spore germ- 

 inating by release of numerous naked protoplasts; these become flagellate zoospores, 

 p, which subsequently encyst; q, the protoplast of each cyst divides to produce four 

 gametes; r, young zygote with the flagella of both gametes. 



