Traditionally, getting that rennet has meant slicing the stomachs of young calves into little pieces dropped into salt water or whey, with something acidic like wine or vinegar used to help draw out the enzymes.

Source: https://www.dailymail.co.uk/femail/food/article-12013635/Foodies-shocked-learn-Parmesan-cheese-not-vegetarian-friendly-using-animal-rennet.html?ns_mchannel=rss&ns_campaign=1490&ito=1490