Abstract
In all of the recent studies (1) on ontogeny of the immune response in mammals, a single antigen has been used to elicit the response in fetuses of one dam. Since individual fetuses in a litter can not be identified with this procedure, the following constraints are imposed: all fetuses must be injected, and with the same antigen; if different antigens are used a second injection can not be given; all sera have to be examined for antibody titers against all antigens used for injection; antibody responses can be compared only between litters, instead of within litters; different doses of an antigen can not be given to littermates; more animals need to be used for any given number of antigens.
Gamma-emitting radionuclides were selected for marking canine fetuses in utero because of their reasonably long physical half-lives (30 to 300 days), easily detectable energy emission spectra, low toxicity to biologic systems, commercial availability, and reasonable price.
