There's evidently no information as to why John Hoyt didn't return to play the Enterprise's doctor, but it's safe to assume he was busy with other projects.
The Cage was the pilot for Star Trek & it's quite common for changes in cast & characters from when the pilot is made to when the show gets picked up as a series.
Within the storyline of Star Trek though, the events of The Cage took place 13 years before Kirk's crew so it's simply an all new crew taking over.
Deforest Kelly
Alexander Siddig.
bones
The star date given in the first regular series episode of Star Trek is 1312.4
Star Trek: The Original Series Star Trek: The Animated Series Star Trek: The Next Generation Star Trek: Deep Space Nine Star Trek: Voyager Star Trek: Enterprise Star Trek: Bridge Commander (TV Series)
Let that be your last Battlefield, Star Trek, Original Series.
The nickname "Bones" is given to Doctor Lenard McCoy - Star Trek (Original Series).
T'Pring was the name of Mr. Spock's fiance in Star Trek: The Original Series.
The best 'original' series episode to show someone who has no idea about "Star Trek" is the episode entitled "The Trouble with Tribbles". It captures the spirt of the original series perfectly.
The original Star Trek ended in 1969 as NBC cancelled the series after just 3 seasons.
The first doctor abord the USS Enterprise was called "Dr. Leonard H. McCoy" and was played by DeForest Kelly. By the way, his nickname was "Bones". :-)
The Star Trek Movie from 2009 was pretty good, but personally I prefer Star Trek: The Original Series and the movies that were made to go along with the original series.
The original series .
The characters' names are Kirk and Spock. And, the series is called: Star Trek The Original Series. Or ST-TOS for short.
The star date given in the first regular series episode of Star Trek is 1312.4
William Shatner .
Star Trek: The Original Series Star Trek: The Animated Series Star Trek: The Next Generation Star Trek: Deep Space Nine Star Trek: Voyager Star Trek: Enterprise Star Trek: Bridge Commander (TV Series)
The first series of Star Trek was the Original series often shortned to TOS airing back in September 1966.
7 Star Trek The Original Series Star Trek The Animated Series Star Trek The Next Generation Star Trek Deep Space Nine Star Trek Voyager Star Trek Enterprise Star Trek Bridge Commander (TV Series)
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