2 M. F. GUYER AND E. A. SMITH 
experiments in this respect, we have imported rabbits from other 
States (Minnesota, Iowa, Illinois, and Indiana) and tested them 
genetically before treating them with serum or crossing them 
with our original strain. 
We have often been asked why we chose fowls and rabbits 
rather than some other forms for our experiments. Fowls were 
used as the source of the sensitized serum mainly because of the 
ease with which they may be kept and handled, and because they 
are not easily infected in surgical operations. Furthermore, it 
was thought that serum from an animal as far removed in rela- 
tionship from the rabbit as the fowl is would perhaps yield a more 
powerful serum than that from a mammal. This opinion, it 
should be noted, is based on statements we have found in books 
on immunity and not on our own experience. In his book enti- 
tled ‘Immunity,’ for example, Citron (Garbat translation) speci- 
fies (p. 144) chickens as among the best animals adapted to supply 
hemolytic sera, and also remarks that “an animal produces a 
better hemolysin the remoter its relationship to the animal from 
which the erythrocytes for injection are taken.”’ 
The use of so foreign a serum as that of the fowl, however, 
doubtless has its disadvantages, since the poisonous, hemolytic 
or general, shock effects arising from the introduction of such a 
widely different serum into the veins -of a rabbit cannot but be 
more severe than if serum from a more closely related form like 
the guinea-pig were used. The frequent severe illness and the 
occasional death which occurred in treated rabbits was probably 
in no small measure due to this factor. Nevertheless, the ad- 
vantages in using fowls seemed to outweigh the disadvantages 
so far that we have continued to use them. 
The availability of the large marginal vein in the ear for intra- 
venous injections is one reason for the use of the rabbit in experi- 
ments such as ours. Also, a doe usually bears from five to eight 
young in a litter and may have several litters in one year. The 
young, furthermore, will begin to breed at from six to eight 
months of age, though this advantage of early maturity is offset 
somewhat by the fact that the litters of very young females 
usually number only three or four individuals. While these 
