While repeating pasteur's experiment with the swan-necked flask, john tyndall occasionally observed the opposite result. tyndall noticed that the boiled broth in the swan-necked flask sometimes did give rise to microorganisms. accordingly, he demonstrated that the broth was contaminated with

endospores

Explanation:

Endospores are structures of bacteria whose function is to provide resistance and ensure the survival of the organism in an inappropriate environment. They have this name because they are produced inside the bacteria. Endospores are produced by bacteria when their environment is unfavorable to their survival. Endospores give rise to other bacteria, so we can say that if Tyndall noticed that the broth boiled in the swan neck balloon sometimes gave rise to microorganisms it means that the broth was contaminated with endospores.