CHYTRIDIALES 91 



endobiotic Endochytrium operculatum, which has an Entophlyctis type 

 of development, has revealed the following situation: The infecting 

 zoospore differs in no essential features of its internal organization 

 from that of Cladochytrium replieatum. After encystment a germ tube 

 is formed which branches and eventually expands locally to produce 

 the incipient zoosporangium. The nucleus, apparently with the nuclear 

 cap persisting, migrates into the germ tube and comes to lie in the 

 sporangial rudiment (Fig. 6 K, p. 88). Initiation of expansion of the 

 axis to form the rudiment is seemingly not dependent upon the pres- 

 ence of the nucleus in the region, since swelling may take place before 

 nuclear migration. Atypical, binucleate, young thalli are occasionally 

 produced. After the establishment of the nucleated reproductive rudi- 

 ment and rhizoidal system within the substratum a period of thallus 

 growth ensues which continues until maturity is reached. Meanwhile, 

 the single nucleus, which in the zoospore was less than 2 \l in diameter, 

 enlarges to 3.5-5 u., without appreciable alteration in its internal struc- 

 ture. The resting nucleus in the young thallus is essentially like that of 

 C. replieatum except that at first no chromatin reticulum can be dis- 

 tinguished. Occasionally, a well-defined nuclear cap is present. While 

 the thallus is expanding, materials are carried back to the reproductive 

 rudiment by the rhizoids. The nucleus, however, remains at all times 

 within the rudiment and just prior to division enlarges to approximately 

 three times its original size. The resting and early prophase stages differ 

 in no respect from those of C. replieatum. Nuclear division in the 

 developing sporangium is simultaneous. The spindles are intranuclear, 

 variously oriented, and bear at their poles densely staining conical 

 structures. The latter are considered by Hillegas to function as centro- 

 somes, since prominent astral rays are frequently found radiating from 

 them. The chromosomes were differentiated before the appearance of 

 the achromatic spindle, but because of their small size their number 

 could not be determined. During the metaphase and succeeding division 

 stages the ring-shaped nucleolus persists, although its size and shape 

 may differ in various nuclei. In the anaphase stages the poles of the 

 spindle protrude through the nuclear membrane. 



By successive simultaneous divisions numerous nuclei are formed 

 in the reproductive rudiment; none of them, however, migrates into 



