AGf~ICULTURAL RESEARCH, PUSA.

464 PRACTICAL BACTERIOLOGY
Several facts emcrgc from the mass of litemtllre OIl the
subJeet :
(1) Certain filterable vimses ean undoubtedly he eultivated
in vitro.
(2) Different conditions of growth are necessary, and these
must be provided just as for the ordinary haeteria, whose
oxygen ann temperature requirenlents, accessory growth
factors (c .• g. for the haemophilie organisms), optimum H-ion
concentrations, etc., must be studied. Thus, in the case of the
common eold virus, anaerobic conditions in living tissues
appear to be lleCessill'.Y; the viruses of herpes and vaccinia
lllultiply in disintegrating 01' slll'viving tissue and active cell
proliferation is not essential, while the fowl-pox, psittacosis
and fowl-plaguc viruses l'equhc !lctively growing tissue cells.
Certain vh'uses grow il'l'espcctive of the nature of the tissuc
eulturcs, hut the viruses of foot-an(l-mouth discase and fowlpox
rcquire definite tissues from certain specics-e.g. the
pails, lips, tongue, 01' hairy skin of the emhryo guinea-pig
[Ire nceessary for the growth of the virus of foot-and-mouth
disease.
Intermediate between Lhe ordinary bacteria and the true
viTllses are the causal organisms of pleuTO-pneumonia of cattle
and agalactia (p. -:LSI). These were formerly ineluded amongst
the filterable viruses, as they pass filters which hold back
ordinary organisms. On the other hand, cultures can readily
be obtained on serum-agar, which, when stained by Giemsa's
method, reveal a great polymorphism of the organism at
diffel'ent stages of growth.
Certain viruses-e.g. of fowl-pox and canary-pox-ean be
cultivated in the cells of the chorio-allantoic membrane of the
developing egg of the hen. Chnntctcristie le8i011s arc produced
in which there are large amounts of active virus.
6. RESISTANCE TO GLYCEROL. - Ordinarv nonsporing
bacteria are killed by 50 per cent. glycerol in
a comparatively short time. Many of the filterable
viruses, on the contrary, retain their viability much
longer in this material than in any other fluid when
kept at 4° C. The preservation of thc vaccine virus
used prophylactically against smallpox is accomplished
by this means. Other viruses which keep for prolonged
periods in glycerol at 4° C. are: poliomyelitis virus, the
virus of febrile herpes (which we have kept virulent
in glycerol for more than one ye.ar), and the :rirm; of
rabies, On the other hand, some VIruses-e.g. VIrUS III