Thranstotheca , a Peculiar Water-Mould. 165 
main hyphae as slender secondary branches (Fig. 53). They are usually 
gently curving (Figs. 49, 52, an d 53 )’ an< ^ do no ^ P resen t the irregular 
crooked appearance which Coker and Hyman ( 9 ) regarded as characteristic. 
As a rule, when the antheridial branches grow through the mycelium, they 
either come in contact with the developing oogonia or lie in close proximity 
to them. When this occurs the branches form antheridia (Figs. 49 to 54)— 
irregular, cylindrical organs, either simple or branched, which curve around 
the oogonium in close contact with its surface. If they fail to reach 
the oogonia, however, the filaments often give rise to zoosporangia (Fig. 52). 
Moreover, antheridia are never formed free, even in cultures containing 
phosphates or other salts, which Klebs ( 18 ) found to stimulate th« formation 
of antheridia on the oogonia. These observations seem to indicate that 
antheridium formation is a response to some contact or perhaps chemical 
stimulus furnished by the young oogonia. This stimulus causes no response 
unless the filaments are very near the oogonia ; but usually the extensive 
branching of the filaments ensures this result. Since the one to several 
antheridia attached to each oogonium are either of androgynous (Fig. 53) 
or diclinous (Figs. 49 to 52) origin in cultures from a single spore, the 
fungus is evidently monoecious. 
Oogonia. The oogonia usually arise somewhat later than the antheri¬ 
dial filaments. Although generally borne in a racemose manner on the 
tips of short lateral branches (Figs. 49 and 53), they may also occupy 
a terminal or intercalary (Fig. 51) position on the main hypha. In any 
case, they are formed by the accumulation of contents, swelling of the 
hypha, and formation of a septum or septa, much as in the case of the 
sporangium initials. The shape, however, is regularly spherical, although 
the intercalary oogonia are occasionally barrel-shaped. At an early stage 
in the development of the oogonia, at times even before the septa have been 
formed, the antheridial filaments become attached and form antheridia 
upon them. When this has taken place the coarsely granular content 
of the latter is modified to innumerable globules embedded in a finely 
granular matrix. The formation of oosphere initials now takes place 
(Fig. 54) by a progressive separation of the contents from the centre 
outward, to form bluntly pyramidal masses which round off and become 
surrounded by walls (Fig. 55). 
The formation of oospheres in an oogonium seems in some way 
correlated with the attachment of antheridia thereto. If no antheridia are 
formed on a young oogonium, it does not form oospheres, but gives rise, 
often repeatedly, to stalked or sessile secondary oogonia by proliferation 
(Fig. 45), or forms sporangia (Fig. 49). On the contrary, when once 
an oogonium has acquired antheridia, it does not give rise to sporangia or 
secondary oogonia, but forms oospheres. Morever, separation of the 
oogonial content into oospheres in no case takes place until after attachment 
