84 



THE SIMPLE HOLOCARPIC BIFLAGELLATE PHYCOMYCETES 



and smooth with one or two exit tubes of varying 

 length which may project slightly or considerably 

 beyond the surface of the host cell. Zoospores bean- 

 shaped with two flagella inserted laterally in a 

 slight depression, and several small refractive gran- 

 ules ; partially formed in the sporangium, emerging 

 in succession, and completing their development in 

 an extramatrical vesicle, or developing completely 

 on the outside within a vesicle, and eventually rup- 

 turing the vesicular membrane. Gametangia formed 

 in the same manner as, and often intermingled with, 

 the sporangia ; antheridia usually of the same size 

 and shape as and alternating with the oogonia ; no 

 differentiation of egg cell, or oosphere and epiplasm 

 prior to fusion ; content of antheridia passing 

 through a pore or small conjugation tube into the 

 oogonium ; fused protoplasts contracting to form a 

 spherical or globose oospore with a thin endospore, 

 a thick smooth, stellate or sculptured exospore ; con- 

 taining one to several refractive globules; germinat- 

 ing by becoming transformed directly into a zoo- 

 sporangium. 



Myzocytium includes at present five species, two 

 of which are doubtful. These species are parasites of 

 desmids, green filamentous, fresh-water algae, 

 nematodes and rotifers in Asia, Europe and North 

 America. The development of the thallus is com- 

 paratively simple. As is shown in figures 3 and 4, 

 the zoospore becomes attached to the host, develops 

 a germ tube which penetrates the host wall and en- 

 ters the lumen of the cell. The content of the spore 

 passes into the tip of the tube, which then elongates, 

 increases in size, and eventually becomes the young 

 thallus. It soon begins to absorb food from the host 

 protoplasm and elongates further into a thick, 

 straight, curved, irregular or lobed structure which 

 mav often extend the full length of the host cell and 

 even into an adjacent one. In the meantime, the 

 zoospore case and part of the germ tube disappear. 

 At this stage the host protoplasm is usually clumped 

 around the thallus and often obscures it, particu- 

 larly in M. proliferum (figs. 6, 7, 9, 16). By the 

 time the thallus is mature most of the protoplasm 

 has been absorbed, so that the host cell is almost 

 empty, except for the parasite and a few extraneous 

 granules. 



In the early developmental stages the thallus is 

 continuous, but as it matures it becomes septate and 

 consists then of a linear series of cells. According to 

 Dangeard (06) it is multinucleate from the early 

 stages on. By further expansion of the individual 

 segments, it usually becomes constricted at the 

 septa, so that the thallus often appears as a series 

 of beads connected by short refractive isthmuses 

 (figs. 5, 19, 37). The segments may occasionally 

 break apart at the septa, giving rise thus to a num- 

 ber of free cells within the host. The zoosporangia 

 are developed directly from the segments of the 

 thallus. In the early stages they usually possess sev- 

 eral small vacuoles, but these generally fuse to form 

 a large central one as the exit tubes develop. In ad- 

 dition, the protoplasm includes a large number of 



conspicuous refractive granules, which apparently 

 break up into smaller and smaller fragments as 

 sporogenesis approaches, with the result that the 

 protoplasm loses some of its refractive appearance 

 and becomes more greyish-granular. 



There is considerable disagreement among stu- 

 dents of this genus as to where the zoospores are de- 

 limited. Many of the early investigators described 

 the protoplasm as emerging through the exit tube 

 and forming an extramatrical vesicle as in Pythium, 

 which then underwent cleavage into swarmspore 

 initials (figs. 10-17). With further development and 

 maturity these become actively motile in the vesicle 

 and are freed eventually by the rupture or deliques- 

 cence of the vesicular membrane. The more recent 

 workers, however, maintained that zoospore rudi- 

 ments are first delimited in the sporangium, pass out 

 singly in succession, and complete their develop- 

 ment at the mouth of the exit tube. That they may 

 even be completely formed in the sporangium is sup- 

 ported by the fact that zoospores may frequently be 

 seen swarming inside. According to these latter in- 

 vestigators a vesicular membrane is visible only dur- 

 ing the initial stages of zoospore emission (figs. 28, 

 44, 45). The process of sporogenesis doubtless va- 

 ries to some degree in the different species, but it is 

 probable that the early workers were somewhat in- 



plate 21 



Myzocytium proliferum 

 (Figs. 1, 2, 5-19 after Zopf, '84) 



Figs. 1, 2. Laterally biflagellate heterocont zoospores. 



Figs. 3, 4. Infection stages. 



Fig. 5. Young thalli in Sjiiroyyra cell; A, unicellular, 

 B, bicellular, and C, four-celled thallus. 



Figs. 6-9. Successive developmental stages of a trieel- 

 lular thallus; fusiform antheridium in center, sporangium 

 on right, and oogonium on left. 



Figs. 10-15. Emergence of protoplasm from sporan- 

 gium, and stages in cleavage and maturation of zoospores 

 within a vesicle. 



Fig. 16. A six-celled thallus, all segments of which de- 

 veloped into sporangia. 



Fig. 17. Reduced Olpidium-like thallus with zoospores 

 in a vesicle. 



Fig. 18. Reduced thallus consisting of an antheridium, 

 oogonium, and oospore. 



Fig. 19. Elongate thallus with oospores. 



M. vermicolum 



24, 27-35 after Dangeard, '06; figs. 25, 26 after 

 '84.) 



Laterally biflagellate heterocont zoospores. 

 , 22. Early germination stages. 

 Elongate irregular germination zoospore on 



Infection. 



Unicellular thallus from nematode. 

 Nematode with a six-celled thallus. 

 Nuclear distribution in a sporangium. 

 Emergence of zoospores. 

 Germination in situ. 



