Answer the following question given this paragraph: 

In some chordates, all four traits persist throughout life and serve important functions. However, in many chordates, including humans, all four traits are present only during the embryonic stage. After that, some of the traits disappear or develop into other organs. For example, in humans, pharyngeal slits are present in embryos and later develop into the middle ear.


Q: In humans, pharyngeal slits later develop into what?


A:
middle ear