Complex sentence: The externa remains immature until a male cypris injects its internal cells into a female's externa, where cypris cells transform into sperm-producing cells within a pair of cypris cell receptacles which were once called the testes.
Split into simpler sentences: The externa remains immature until a male cypris injects its internal cells into a female's externa. Injected male cells migrates into a pair of cypris cell receptacles which was once called the testes.