You're likely referring to Spallanzani's 1768 broth experiment, not a 1768 experiment by Needham. While Needham's experiment in 1748 is often cited, it was Spallanzani who countered his findings.

Here's the breakdown:

* Needham's 1748 experiment: Needham boiled broth and sealed it in flasks, then observed microbial growth. He argued that this proved spontaneous generation (life arising from non-living matter).

* Spallanzani's 1768 experiment: Spallanzani challenged Needham's results by boiling broth for longer periods and sealing the flasks more tightly. He observed no microbial growth in these flasks. He argued that Needham's experiment didn't kill all microorganisms, and the air entering the flasks after boiling could have introduced new ones.

So, where did the organisms in Needham's experiment come from?

* Pre-existing microorganisms: The broth likely contained pre-existing microorganisms that were not completely killed by Needham's boiling process. These microorganisms then reproduced and became visible.

* Contamination from the air: The air entering the flasks after boiling could have carried microorganisms, contaminating the broth.

Spallanzani's work was crucial in challenging the idea of spontaneous generation and paving the way for Pasteur's famous swan-neck flask experiment, which finally proved the importance of sterilization and the role of pre-existing organisms in biological growth.