Thraustotheca , a Peculiar Water-Mould. 167 
becomes transformed into a spherical body 22 to 24 /x in diameter, contain¬ 
ing a loose protoplasmic reticulum (Fig. 60). A hypha (Fig. 61) is now 
sent out, which may, after limited growth, form a sporangium, or may 
continue to grow into an extensive mycelium depending on the amount of 
nutriment stored in the spore or present in the surrounding medium. 
In pure water sporangia are formed, either directly or on short hyphae 
(Fig. 63) ; but if nutriment is added, a more or les£ extensive mycelium 
arises (Fig. 62). Since the heavy wall of the oogonium persists in pure 
cultures, the oospores germinate in situ ; and although in some cases their 
expansion is sufficient to rupture the enveloping wall, in general the hyphae 
of germination grow out through the pits, or follow along the empty shells 
of the persistent antheridia until free (Figs. 62, 63, and 64). 
Discussion. 
The development of Thraustotheca presents several points of interest. 
Spore liberation. The method of sporangium dehiscence and spore 
liberation which characterizes the genus is at present unique among the 
Saprolegniaceae. In no other member of the family do non-motile sporan- 
giospores escape by bursting the sporangium wall. Moreover, none of the 
theories advanced in explanation of spore liberation in other genera seem 
applicable to the process in Thraustotheca. The theory that the sporangio- 
spores escape by their own motility is not corroborated, since they are 
consistently non-motile and without cilia. Even in the better-known 
genera, however, this theory is not universally supported. In Saprolegnia , 
Leptolegnia , and Pythiopsis the sporangiospores are undeniably ciliate and 
motile. Humphrey ( 15 ), however, believed that-an internal force other 
than spore motility was involved in emptying the sporangia ; and the writer 
has observed a number of cases which corroborate this point of view. 
In Pythiopsis cymosa } de. Bary, for instance, dissolution of the escape- 
papilla is occasionally delayed until the sporangium becomes so distended 
by the swelling of the spores that it finally bursts at the tip, expelling the 
zoospores a considerable distance with such violence that they seem stunned 
and remain quiet some minutes before swimming away. Moreover, although 
the sporangiospores of Aphanomyces escape from the sporangium and 
group in a hollow sphere at its mouth, both de Bary (2 and 4 ) and Rothert 
( 24 ) found that they lack demonstrable cilia. In the case of Achlya certain 
investigators have asserted that the escaping sporangiospores are ciliate, 
while others have maintained that they are not. Cornu (10) first claimed 
that the sporangiospores are ciliate; and this has been corroborated by 
Hartog ( 12 ) for Achyla polyandra , Hild., and A. recurva , Cornu ; by 
Humphrey ( 15 ) for A. americana , Humph.; and by Coker (8) for A. para - 
doxa , Coker, and A. caroliniana } Coker. On the other hand, Rothert ( 24 ), 
Horn ( 14 ), and Coker ( 7 ) deny the existence of cilia on the sporangiospores 
