68 
to the diphtheria toxine. Such hereditary influences, which would 
seriousl}" afl'ect the results, should be guarded against by elimination 
of this stock. 
Wetcfld of the guinea pig. — Ehrlich prescribes the use of guinea pigs 
weighing 250 grains. 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 activit}^ of metabolism. We have found 
young hungry pigs to eat as much as 35 grams in a da}^ The weight of 
a 3 ^oung pig will ordinarily change from 10 to 20 grams in a day, 
depending upon the food and water taken. AVe, 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 first 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. Occasionalh^, when the mother gives birth to onh" one, 
we have found, under such favorable conditions, that this single 
ofl'spring may gain so rapidh" as to weigh 250 grams before it is four 
weeks old. 
We do not alwa}^s have enough guinea pigs of our own raising 
to carry on the work. In that case we bu\^ from reliable dealers, 
insisting on pigs weighing less than 225 grams, so that we 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 suflicientl^^ accurate, weighing to 2.5 grams, and are much quicker 
and more convenient than pan balances with weights. 
As soon as the 3 "oung pigs are old enough to be weaned the 3 " are 
divided, separating the males from the females. Pregnant female pigs 
should not be used in this work. 
A pig having been selected and weighed has a card made for it with 
a rough diagram, on which is indicated its pecLiliarities of color or 
other marks. This is a satisfactoiy method of identifying the animal. 
The character of card is here shown (fig. 16). 
Technigue (f operation. — The abdomen is shaved the da 3 " before 
opei’ation, clearing the site of operation so that Ave ma 3 " readih^ see 
just what we are doing. An area about 2 inches in diameter is shaved. 
The preparation of the animal card shown in hg. 16, the shaving, 
etc., is done the da 3 " before the operation, so that the pig is not 
handled more than necessaiw to receive the injection on the day of 
the operation. 
The pig is held b 3 ^ an assistant and the needle introduced in the left 
flank from one-half to an inch above the supraspinous process of the 
