215 
on the Saprolegnieae. 
metabolism that from all analogy we must infer goes on in 
the maturation of the zoospores. The experiment is not 
conclusive. 
II. The zoospores on emission in a similar experiment are 
not attracted when a cleft for air is left, nor when there are 
air bubbles. This is quite possible according to Fechner’s 
law ; from zero to a very small quantity of air the attrac- 
tion may be more marked than from a small quantity to 
saturation. 
But I admit that my reasoning went too far in definitely 
ascribing the exit of the zoospores to positive aerotaxy. The 
facts are equally ascribable to what I may term i negative 
pneumatotaxyl or the escaping from the products of their own 
metabolism. Some preliminary experiments lead me to think 
however that carbon dioxide is not the stimulating substance. 
Rothert has repeated Walz’s experiments, which tended to 
show that liberation was due to an expulsive substance, treat- 
ing the zoosporangia at the moment of liberation with syrup 
(twenty-five per cent, cane sugar) or glycerine. He found first 
that the motion of the free zoospores is arrested by these, but 
does not recommence on dilution, though they retained their 
power of germination. On the sporange the effect was peculiar 
and apparently irreconcilable with either theory. On adding 
a drop of the reagent, liberation stopped and soon recom- 
menced ; the same sequence occurred on adding a second drop, 
&c. ; finally it stopped, not to recommence even on dilution. 
This is certainly conclusive against the expulsive substance ; 
but I fail to see how it tells at all against my views, as the 
free zoospores are also arrested in the reagent. 
Rothert has confirmed my absolute disproof of the existence 
of an expulsive substance ; for after the arrest of liberation by 
the action of iodine or alcohol, on dilution no further liberation 
takes place. He concludes that on the whole his experiments 
tell rather in favour of spontaneous liberation. But he is 
met by the difficulty that all the reagents behave in the same 
way to Achlya as to Saprolegnia , while for Achlya he cannot 
admit the possibility of any but an expulsive mechanism. 
