Clone Cadets - S3-E1
Trivia: Commander Colt watches Domino Squad do their test and says, "Start The Citadel challenge, version THX, variable 1138," which is a reference to THX 1138, another film by George Lucas.
Trivia: When Sy Snootles kills Ziro the Hutt, he mumbles "What a world," a reference to The Wizard Of Oz.
Answer: Dooku never accepted Ventress as his apprentice, as far as I know; he just kept her as his devoted personal assassin. In Phantom Menace, it is stated that there can be only two Sith at a time, to prevent a power struggle in the lower ranks. To accept Ventress as an apprentice, Dooku would have violated that rule.