118 W. ZOPF ZDR KENNTNISS DEB I'lIYf OMYCETEN. 



as many as 10). The medium-sized usually possess from three to 

 five, sometimes only two, whilst in the smallest one to two at the 

 most ocenr. Their position is a fixed one, inasmuch as it always 

 corresponds to the girdle-band (front) view of the diatom cell. 

 Sometimes they are arranged in a single, sometimes in a double, 

 row. In the latter case one row corresponds to the anterior, the 

 other to the posterior surface of the girdle-band. 



That this is the case is most readily understood by observing the 

 valve-view of the Synedra; the excretory ducts will then be seen 

 lying to the right and left. 



Moreover, the excretory ducts are distinguished by the fact that 

 they are of a much stouter texture as compared with the extremely 

 delicate wall of the sporangium ; ami, as is shown by the use of a 

 solution of iodised zinc-chloride, they consist of cellulose. 



It is further remarkable that the excretory ducts are not com- 

 pelled to perforate the siliceous membrane. The pressure, namely, 

 exerted by one or more parasites as they increase in girth, on the 

 two valves of the host-cell, causes them to bend outwards, so that 

 the Synedra now, in a girdle-band view, appears broadest in the 

 middle ; finally, the frustule dehisces entirely. This process is 

 completed before the excretory vesicles of the sporangia are ready 

 to force themselves through the siliceous membrane (if they are 

 able to do so at all) — indeed, often before the vesicles have arrived 

 at maturity — so that any necessity for penetrating the membrane is 

 avoided. Of course, it not infrequently happens that the sunder- 

 ing of the valves does not take place sufficiently early. But even 

 in these cases the siliceous membrane is not perforated by the 

 vesicles. The latter then either do not open at all (and this is not 

 uncommon), or the swarm-spores are evacuated into the closed 

 diatom cell. Instead, however, of these imprisoned zoospores 

 perishing, as is the case in most of the Chytridiaceas, they, pro- 

 vided of course the host-cell still contains a sufficiency of nutri- 

 ment, grow to new parasites, having burst or crushed the 

 degenerated sporangial membrane. These non-immigrated para- 

 sites, in consequence of their numbers and unfavourable food con- 

 ditions, always remain in a correspondingly rudimentary condition, 

 and either retain their spherical form or, at most, become ellipsoidal 

 or spindle-shaped. They develop into sporangia. The fact that 

 numerous rudimentary parasites may be present in the long 

 sporangial sacs, loses its singularity when we are acquainted with 



