44 PROGRESS OF MICROSCOPICAL SCIENCE. 
greater number than was anticipated—and Mr. Hart’s collections at 
lower latitudes are both rich and interesting. The cryptogamic 
flora was of course much more varied and abundant. 
The Relation between Activity of Nerves and the Size of their 
Sheath.—Dr. R. Saundly (‘Medical Record’) reports that Herr Dr. 
K. Arnt states that a comparison of the facts derived from anatomical 
observations, human and comparative, and the results of clinical and 
pathological investigations, shows that the development of the 
medullary sheaths stands in direct proportion to the activity of the 
nerve-fibre. In the lower animals and in the embryo, many nerve- 
fibres are naked, and the development of the medullary sheath takes 
place by the formation of small round cells (Kugelchen), which 
arrange themselves around the nerve-fibre, and finally melt together 
to form a homogeneous sheath, which thickens by additional concen- 
trically formed layers. He considers its formation to be due to 
functional irritation. 
Fungi parasitic on Corals.—Dr. Duncan, F.R.S., has given an 
interesting paper on this subject to the Royal Society,* and we should 
have endeavoured to obtain it for publication but for the great expense 
of reproducing the plates. The following is a summary of the facts 
elicited by the author’s researches: Quekett, Rose, Wedl. Kélliker, and 
Moseley have noticed and described the borings of vegetable parasites 
in molluscan shells, fish-scales, and corals; but no special attention has 
been paid to the filaments penetrating the last-mentioned organisms. 
Corals from the littoral zone down to 1095 fathoms are frequently the 
seat of the parasitic growth of two kinds of Achlyew, whose horizontal 
range is from Davis Straits to the tropics and 15° S. lat. Fossil 
corals of Silurian age were also affected by closely allied, if not speci- 
fically identical, growths. The method of investigation is by making 
thin sections of the sclerenchyma, and also by dissolving out the carbo- 
nate of lime. The parasites are filamentous, and fill up the canals which 
they form; they resemble a mycelium, and penetrate the coral, living 
upon the organic basis, and having their length, breadth, and straight- 
ness, or branchings, dependent on the peculiar nature of the arrange- 
ment of the spicula in the different species of the Madreporaria. The 
entry is made from oospores, zoospores, and by the accidental contact 
of the parasites whilst perforating alge situated on the wall of the 
coral; and the penetration and growth appear to be the combined 
results of the formation of a soluble bicarbonate of lime by the action 
of carbonic-acid gas evolved from the growing end of the tubular 
filament, of the pressure incident to growth, and of the movements of 
the cytioplasm and the cell-wall. The vegetative life of the parasites 
is accompanied by reproductive efforts within the corallite; for the 
aggregation of granules within the viscid transparent cytioplasm can 
be detected, and their formation into large conidia and into small 
unciliated zoospores also. Following the peculiar physiological habit 
of the Saprolegnian group of Achlyw, the reproductive elements ger- 
minate and produce either large or very small tubes which, after 
penetrating the parent cell-wall, get through the solid investment, 
* «Proc. Roy. Soc.’ No. 174. 
