802 
ROSS GRANVILLE HARRISON 
details of manipulation during operations is so fatiguing, that only 
a small number of preparations can be made in one day. Many 
preparations proved to be absolutely sterile. In some of these 
the tissues were kept alive for over five weeks, and in a great many 
for one or two w^eeks. Some were contaminated, most frequently 
with Bacillus subtilis, but even in these cases the organisms did 
not usually appear in sufficient number to injure the living tissue 
until after it had been kept under observation for four or five days, 
which was long enough for the present purpose. Several epi- 
demics of mould (Penicillium) were encountered, but this too grew 
slowly, usually from a single spore or two, and as it does not seem 
to kill the embr^'onic cells, it interfered but little with observ^a- 
tions. 
The tissue to be studied is dissected out from the sterilized em- 
bryos in a small flat dish containing dilute salt solution. After 
this is done the next step is to obtain a drop of lymph from the 
frog, which has already been prepared. The animal is suspended 
or placed in an upright position, and after cutting into the lymph 
sac near its upper end a long fine pipette is introduced and a small 
drop is drawn from the bottom of the sac. This is then placed 
upon the cover-slip, and the piece of tissue is quickly transferred 
to the lymph by means of the same pipette, with care to take with 
it as little of the salt solution as possible. Then the cover-slip 
is inverted over a depression slide and the preparation sealed by 
means of paraffine. It is important to have the depression in the 
slide deep enough to prevent the drop, which must also be small, 
from coming into contact with the bottom. 
The procuring of the lymph is the most difficult part of the 
whole procedure, and the variability in its quality and in the 
amount obtainable, introduces into the work an element of in- 
constancy, which is a serious disturbing factor, preventing, as 
it does, a sharp clear-cut process of experimentation with exact 
controls from being carried out. The composition of the lymph 
varies not only amongst individual frogs but also in the dif- 
ferent lymph sacs of the same individual, according to the 
position in which the animal has lain, the time since anaestheti- 
zation, and other factors of unknown nature. In general it may 
