310 Cc. C. LITTLE 
oculation. In this way they present no unfamiliar odors to the 
old mice and are therefore accepted by them in a majority of 
cases without further investigation. If these precautions are 
omitted, one finds not infrequently that the old mice in the pen, 
. not recognizing the returned young one, will neglect it or will 
bite and perhaps devour it. 
b. Observation and record 
Beginning with a date two weeks after inoculation, weekly 
observations are made upon all inoculated mice. The mice are 
examined individually by palpation, and the presence or ab- 
sence of a mass noted. Each mouse is given a serial number, 
and is in this way kept distinct from all others. If a mass is 
present, it is described, and if it is larger than a pinhead a sketch © 
is made of it on the record sheet of the mouse. 
Thus weekly observations are made in the case of all animals 
up to and including the sixth week after inoculation. From that 
time on, observations are made upon only those animals 
showing a mass. In this way a diagrammatic representation 
is obtained of the gradual growth of the tumor as well as a 
record of its diminution and eventual disappearance, should 
this take place. The absolute size of the mass cannot, of 
course, be very accurately determined. The observer estimates 
it as carefully as possible and represents it by an outline upon 
the record sheet of the mouse. An effort to diminish the 
influence of the personal equation has throughout been made 
by utilizing only a small number of trained observers who 
frequently compare their impressions as to the size of the mass 
recorded, and in this way reach a fairly accurate estimate. It 
is doubtful whether efforts to make these observations more 
accurate would be of real value since the variation in the amount 
of hair and thickness of skin in individual mice would undoubt- 
edly to some extent influence the measurable size of the mass, 
even were measurements taken with calipers. 
Furthermore, the nature of the mass itself, including, as it 
often does, large areas which are cystic and collapsible upon 
