Correct me if I'm wrong, but don't Argonians have to lick sap off of a Hist tree when they are young in order to be capable of things like basic thought and walking on two legs? And since Hist only grow in Black Marsh, doesn't that mean that all Argonians would HAVE to have been born there?
Good question.
It is unkown how far the symbiosis between the Hist and the Argonians goes, but safe money is on a deep connection.
We know that Argonians in Black Marsh lay eggs, but those who are far from the Hist and in more inhospitable places give live birth, presumably to give the offspring a better chance of survival in harsh surroundings.
(Mark Nelson:
Argonian egg-laying, again (11/10/03)
Never underestimate the adaptability of Argonians, or, more specifically, the power of the Hist to allow Argonians to adapt.
I wouldn't expect to hear an Argonian born in Skyrim (or on Solstheim, for that matter) mention being hatched. Nor would I expect to hear more transient Argonians (say, members of a small, nomadic tribe) speak about laying eggs. However, in warmer climates, in places with established, stable, and permanent communities, you would likely see a great number of eggs.)
Tavelling-New-Woman from Morrowind gave some insight into your question with her dialogue: "
My parents told me not to lick the Hist tree until my Naming Day, so I did. My parents caught me and a boy from my nest in a soft-leaf patch right before the naming ceremony. I feared I'd get more of a licking from my parents than I gave the tree... So I ran as fast as I could through the swamp and into the rest of the kids waiting for their naming ceremony. Everyone had a good laugh and the name stuck."
This text indicates that it is not sentience that the Hist sap provides, but sixual maturity.
It is possibly I suppose that Argonians out of Black Marsh carry a supply of Hist sap for their ceremonies.