Why is Utterson concerned for Dr. Jekyll after reading his will?

Mr.
Utterson is concerned after reading Dr. Jekyll's will because his friend is leaving everything
to Mr. Hyde, a man Mr. Utterson has never heard of or met. This in itself is strange, because
Mr. Utterson and Dr. Jekyll go back a long way and come from the same set of friends. Utterson
also worries because the will stipulates that should Dr. Jekyll disappear for more than three
months, Mr. Hyde should:

. . . step into the...