CHYTRIDIALES 87 



flagellum (Fig. 5 A). Cytological details of the sexual process have al- 

 ready been given (see under "Sexual Reproduction," p. 75). 



In the endophytic polycentric Cladochytrium replication (Karling, 

 1937b) the infecting zoospore has a large central nucleus (Fig. 6 A, 

 p. 88). Within it is a small nucleolus and, outside the nuclear mem- 

 brane, a large deeply stained nuclear cap of uncertain origin and com- 

 position, similar to that in Blastocladia, Allomyces, and Monoblepharis. 

 Faint cytoplasmic strands connect the nucleus to the posterior flagel- 

 lum. After encystment a penetration tube is formed, through which 

 the nucleus and possibly the nuclear cap as well, if this has not already 

 disassociated, pass into the substratum (Fig. 6 B-C). In some individuals 

 the tip of the tube expands at once to form the first turbinate cell, or 

 spindle organ, into which the primary nucleus migrates. In others 

 formation of the first spindle organ may sometimes be delayed until 

 the now strongly elongated nucleus has reached the tip of the tube. 

 Recently migrated nuclei are readily recognized by their distinctly 

 pyriform shape (Fig. 6 D). In them the chromatin forms a faint retic- 

 ulum and the deeply stained nucleolus is disposed in a broad flat 

 transverse ring around the inner periphery of the narrower end. One 

 or several rhizoids then emerge from the surface of the primary spindle 

 organ and eventually form at their tips new spindle organs. As the 

 primary swelling increases in size the nucleus divides mitotically, with 

 the formation of an intranuclear spindle. During such division the 

 nucleolus persists as a laterally placed, dark-staining, generally arc- 

 shaped structure. Cytokinesis follows, and the spindle organ becomes 

 two-celled. The more distal nucleus subsequently migrates into the 

 secondary rhizoid and eventually comes to lie in the new spindle organ. 

 Sometimes both nuclei migrate from the primary spindle organ before 

 the formation of the septum. The mature thallus thus "replicated" 

 is multinucleate, with the nuclei present (save when migrating) only 

 in the spindle organs and rudiments of the reproductive structures. In 

 the rhizoids the cytoplasm is strongly vacuolate and bears, at irregular 

 intervals, deeply staining granules of unknown composition. 



The resting nucleus of the thallus is large and spherical, oval, or 

 citriform. Lying to one side near the inner wall of the nuclear membrane 

 is a conspicuous disclike, oval, or bandlike nucleolus entirely similar 



