68 
to the diphtheria toxine. Such hereditary influences, which would 
serious!}" affect the results, should be guarded against elimination 
of this stock. 
Weight of the gidnea pig . — Ehrlich prescribes the use of guinea pigs 
weighing 250 grams. We consider a pig weighing anywhere from 
250 to 280 grams as complying with this requirement. 
The weight of a young guinea pig varies with the quantity of food 
and water taken and the activity of metabolism. We have found 
young hungiy pigs to eat as much as 35 grams in a daj". The weight of 
a young pig will ordinarih" change from 10 to 20 grams in a da}", 
depending upon the food and water taken. We, therefore, always 
weigh our guinea pigs in the morning before feeding time. We some- 
- times use guinea pigs weighing more than 280 grams, but less than 
350 grams, for our tirst work on a toxine or serum to determine its 
approximate strength. 
In our experience' a guinea pig weighs about 100 grams at birth, 
and is between six and eight weeks old before it weighs the required 
250 grams. Occasionally, when the mother gives birth to only one, 
we have found, under such favorable conditions, that this single 
offspring may gain so rapidly as to w"eigh 250 grams before it is four 
Aveeks old. 
We do not always haA"e enough guinea pigs of our own raising 
to carry on the w"ork. In that case we buy from reliable dealers, 
insisting on pigs weighing less than 225 grams, so that Ave never use 
a pig that has not been in the laboratory under observation ten days 
or two weeks. 
The pigs are weighed upon a spring-balance scales. These scales 
are sufficiently accurate, Aveighing to 2.5 grams, and are much quicker 
and more convenient than pan balances Avith Aveights. 
As soon as the young pigs are old enough to be Aveaned they are 
divided, separating the males from the females. Pregnant female pigs 
should not be used in this work. 
A pig haA"ing been selected and weighed has a card made for it with 
a rough diagram, on which is indicated its peculiarities of color or 
other marks. This is a satisfactory method of identifying the animal. 
The character of card is here shown (tig. 16). 
Technupie of operation . — The abdomen is shaA"ed the day before 
operation, clearing the site of operation so that Ave may readily see 
just what Ave are doing. An area about 2 inches in diameter is shaA"ed. 
The preparation of the animal card show n in tig. 16, the shaving, 
etc., is done the day before the operation, so that the pig is not 
handled more than necessary to receive the injection on the day of 
the operation. 
The pig is held by an assistant and the needle introduced in the left 
flank from one-half to an inch above the supraspinous process of the 
