The general notion of a direct limit of a commuting system of embeddings, indexed by pairs in a directed set, has seen heavy use in set theory. It is the same notion as in category theory. I was surprised to find that the general definition does not appear in the book on model theory by Chang and Keisler ([MR1059055](https://mathscinet.ams.org/mathscinet-getitem?mr=MR1059055)). Who was the originator of this idea?