CHYTRIDIALES 103 



faintly staining, poorly defined reticulum. Nuclear division takes place 

 first within the thick-walled zoospore cyst (Fig. 9 B-D) and the 

 daughter nuclei migrate into the growing germ tube (Fig. 9 E-F). 



At prophase, the nucleus contains a lens-shaped, deeply staining 

 nucleolus and several discrete bodies which may possibly be regarded 

 as an early stage in chromosome formation. At metaphase (Fig. 9 

 E-G), the chromosomes form a dense band on the intranuclear spindle 

 which, in polar view, appears as a ring on the equator (Fig. 9 F). 

 Individual chromosomes are difficult to recognize because the chro- 

 matin is compact and the nucleus small. No centrosomes were seen 

 nor was there any evidence of astral-ray formation. Nuclear divisions 

 in the germinating zoospores, developing thalli, and rhizomycelium 

 are not synchronous (Fig. 9 F), in contrast to those in the developing 

 sporangia which are synchronous. 



The sporangia originate as swellings on the broader parts of the thal- 

 lus, either alone or in groups. A single nucleus migrates into each swell- 

 ing, which is then cut off by a cross wall from the thallus. In the in- 

 cipent zoosporangium, division of the primary nucleus is immediate 

 and is not preceded by a preliminary enlargement. Sporangial mitosis 

 is synchronous (Fig. 9 I-J), as in all other chytrids thus far investi- 

 gated and contrasts with the nonsynchronous nuclear division in the 

 Blastocladiales. The resting nucleus contains a dense spherical nucleo- 

 lus and a weakly staining chromatin network. During prophase, chro- 

 matic strands (Fig. 9 K) develop which later contract to form the 

 chromosomes (Fig. 9 L-M). Equatorial-plate stages (Fig. 9 G, J) 

 show intranuclear spindles which are rather broad at the center and 

 taper at the poles. The origin of the spindle remains obscure. At 



spindle. Q. Feulgen-stained sporangium showing Feulgen-positive anaphase 

 chromosome masses, nucleoli unstained. R. Multinucleate sporangium. S. 

 Multinucleate sporangium showing aggregation of chromatic particles about 

 the nuclei. T. Coalescence of basophilic granules about the sporangial nuclei. 

 U. Beginnings of progressive cleavage in a sporangium, nuclear caps fully 

 formed. V. Polygonal zoospore initials in sporangium following cleavage, 

 nuclear caps fully formed. W. Enlarged group of zoospore initials showing 

 granules surrounding nuclei. X. Part of a thallus showing organization and 

 cytology of the vegetative system (composite drawing). 

 (All from Ajello, 1948a) 



