Complex sentence: Each animal could be used only once, though several procedures regarded as part of the same experiment were permitted, and the animal had to be killed when the study is over, unless doing so would frustrate the object of the experiment.
Split into simpler sentences: Each animal could be used only once, though several procedures regarded as part of the same experiment were permitted. And the animal had to be killed when the study is over, unless doing so would frustrate the object of the experiment.