4 Psyche [February 
haps the early stages of the virus require some particular organ or 
tissue or some particular condition. The insect itself fulfills the 
required condition, but the blood cells growing in vitro do not. 
The later stages of the virus, however, find the conditions suitable 
on the tissue culture slides. 
Then again I may not have cultivated the virus at all. The 
caterpillars were infected with the polyhedral virus which may 
have a strong affinity for some particular tissue other than the 
blood. ‘Toxins may be elaborated and getting into the blood may 
start the degenerative changes which culminate in the formation 
of polyhedral bodies. These degenerative changes, after beginning 
within the animal, may later proceed outside of it on the tissue 
culture slides in the absence of the virus. I think that a series of 
passage infections would clear up the whole matter. This I have 
not yet attempted. A series of animals should be infected with 
fresh virus. In ten or twelve days tissue culture slides should 
be prepared from the blood. When polyhedral bodies begin to 
form, another series of animals should be infected from the slides. 
In ten or twelve days the blood should be taken from these ani- 
mals and kept on slides and if polyhedra form, a fresh series of 
animals should be again infected and so on. Such a series must, 
_ of course, be accompanied by suitable checks. If the animals 
in the later experimental series die typically and if there is no 
increase in the period from infection to death (about twenty days) 
it would be fairly certain that the virus has been cultivated and 
that one is not dealing with a partial recovery of the amount of the 
virus originally used. 
In cultivating insect tissue it is always well to prepare a great 
many slides. A few become contaminated with bacteria, but 
many disintegrate normally without showing the least inclination 
towards growth. The ability of the tissue to grow well also seems 
to a slight degree to vary according to the species of insect. The 
tent-caterpillar blood, for instance, does not grow as readily as the 
blood taken from the true army or fall-army worm. The blood 
from these two species does not grow nearly so well as the blood 
of the gipsy-moth caterpillar. One should never discard slides 
for at least a week or more. Very frequently nearly all of the 
cells will disintegrate during the first five or six days. A few, how- 
ever, live and these later increase and multiply forming beautiful 
