2 1 6 Hartog . — Recent Researches on the Saprolegineae . 
This is obviously due to his having overlooked the cilia, and 
cannot weigh at all in the matter. 
I may here point out that the aggregation of the spores in 
Achlya into a hollow head at the mouth of the sporange they 
have just left, appears to be due to the mutual attraction of 
the spores and the tendency to place themselves with their 
axes parallel. This is visible even in the sporange, and in- 
duces the aggregation into a cylinder or gut-shaped mass in 
poor sporangia, and materially interferes with their final 
separation. When they leave the sporange this is counter- 
balanced by that peculiar irritability (‘ negative pneumato- 
taxy’?) which determines their exit. This mutual attraction, 
which I may term adelphotaxy , can only act at a short 
distance ; when the sporange is discharged near the margin of 
the hanging drop, or in a thin layer of water on a slide, we 
constantly see single spores escape from the mass, swim away, 
and encyst apart. Cases of adelphotaxy are not so rare as 
we might think ; in the embryology of animals this form of 
irritability is implicitly assumed by every one. In the vegetable 
kingdom we find it most obvious in the Pediastreae. 
This paper is not final; it is obvious that while I have 
shown that the liberation is due to irritability of the zoospores, 
and is probably induced by a chemical stimulus, we are still 
in the dark as to whether this stimulus is really the positive 
one of oxygen in the medium (aerotactic), or the negative one 
of the soluble products of the metabolism of the zoospores in 
the sporangia themselves (pneumatotactic). Moreover there 
are numerous processes of differentiation in Achlya which I 
am now studying, and which will, with the completion of my 
researches on the nature of the liberative stimulus, form the 
subject of a fresh publication. 
We are indebted to Rothert for the discovery that frag- 
ments of a healthy culture of Saprolegnia may be cut off and 
will continue to thrive in the hanging drop, and are much 
more normal than the fly-leg cultures usually worked with. 
I have found garden-centipedes far more suitable for large 
cultures than meal-worms. 
