I get the fact hes a powerful mage and is four thousand years old, but what kind of school of magic would allow him to do that? I'd guess Alteration or Illusion, also is it ever explained how and why he did it?

Alchemy perhaps.

I think this is one of those times that it's important to remember that the schools of magic are constructs. They're a way for mortals to categorize magic, rather than any hard set rules.

Beat me to the punch...

Um... http://www.empowernetwork.com/kenyattaboone/files/2012/10/how-to-train-a-parrot-to-talk.jpg.

It definitely was a long, complex process involving spells from many, if not all, of Galeiron's schools.

Seems to me it would be a combination of Restoration, Conjuration, Alteration, and Mysticism. Maybe a bit of Enchantment and Alchemy, for specialized equipment.

So in other words, a blend of most of them that renders the constructed schools moot.

Wait, what are we talking about?

Divayth Fyr's four daughters/wives who are actually female clones of himself.

The school of AWESOME. That's what school.

Not just clones, Dragonslayer. Aspects of him.

Thaumaturgy.

Every one likes to forget about that school

Yeah, it wouldn't surprise me if there was some of this involved.

That's my favourite school, actually, but it doesn't seem to work for Divayth's purpose... http://www.uesp.net/wiki/Lore:Thaumaturgy

This is what Divayth Fyr has to say on the topic of his daughters:

So... he was studying divine cancer, and at some point discovered something that let him grow people in vitro. It sounds like he discovered stem cells, and then figured out how to make induced pluripotent stem cells, for a start. And then how to start the stem cells differentiating like embryos do naturally. Or the TES equivalent. If you had to stick the whole process under just one school of magic, which I probably wouldn't, I'd categorize it under Mysticism. Or necromancy. Is necromancy a subset of Mysticism, or its own school?

Vuhon, a very knowledgeable Dunmer mage who built the Ingenium and later became Lord Umbriel, also figured out a way to grow people from little flesh pods in his pocket realm of Oblivion. This may have involved Daedric magic though. He was essentially a Demiprince and was reusing souls the way the Daedra souls are generally recycled, except he could even do this with mortals.

I don't think it was Vuhon who figured all that out. The way I read it, it was the ur-Hist of Umbriel doing what Hist do.

This.

Vuhon only had power through them.

On a related note, I think the ur-Hist of Umbriel offer some great insights into the Argonian/Hist relationship. People have speculated that the Hist created the Argonians but Lord of Souls/Infernal City make me believe that the Argonians were just incorporated into the Hist's domain (thus they have a measure of control over them equal to the Umbriel-Trees over the Umbriel Citizens.)

I reject the idea that the Argonians used to be normal lizards.

They must come from somewhere and if they are not Hist they are Ehlnofey.

In any case, Mere-Glims dialogue on the matter of rebirth seemed to indicate to me that they used to be non-sapient until the Hist intervened.

From TIL:

The following is a description of what Glim experiences when he is dead. His soul is trapped in a soul gem and being reborn into a new body grown by the Umbriel-Hist (Um-Hist henceforth). It is likely to resemble what one would experience with traditional Black Marsh Hist

“[Glim] swam in black water, probing through the rotting leaves, lifting his eyes now and then above the surface to search the shallows and shore for movement. Larger things in the depths of the swamp couldn’t reach him here, amidst the twisting cypress roots; here the danger usually came from land.
Something in the mud moved, and he snapped at it with webbed paws and lifted a feathery-gilled wriggler into view. He ate it happily and searched for more, but in a short time his belly was full and he felt like basking. He swam lazily back to the gathering hole.
The old ones had already claimed the choicest perches, so he crawled onto a log already crowded with his siblings and wriggled down among them until he felt the rough bark against his belly. When his brothers and sisters gave up their sleepy, halfhearted complaints at his added company, he felt the sun on his skin and began to dream his life; swimming, basking, killing, avoiding death, the sun and moons, all mystery, all terrifying, all beautiful. Each day the same day, each year the same year.
Until the root came, and the taste of sap. Some changes were slow, others came quickly, and he—they—flowed together, found the stream of time. His old body wasn’t forgotten, but it changed, became more like things the root remembered from otherwhere; his hind legs lengthened and his spine stood up. Small thoughts in his head put out branches, and those branched also, until what had before been warmth, light, shadow, movement, fear, contentment, anger, and lust became categories instead of simple facts. The world was the same, but it seemed more, bigger, stranger than ever.
Death followed life and life death, but it all flowed through the root, each life different, each the same.
Until that, too, ended, and the root was ripped away, and he was alone. The gathering place was empty except for him—no elders, no siblings. He swam in black water, forgetting everything. Losing his form, melting away.
But in that dissolution, the illusion was also dissolved. He was many, and he was one. He sang, a plaintive tune, a remembrance, a prayer. All of his voices took it up, trembling it out through every branch and root, through heart and blood and bone.”

Very interesting. I had forgotten about that bit actually (been a while since I read the books.)

Vuhon certainly took advantage of and was aware of the powers of his Um-Hist. I'd assume that as he integrated them into his pocket realm and had absorbed some of Vile's mind he was semi-omniscient and could noodle it out for himself.

The ingenium was always conceived as parasomnoconductive, for which neither Vuhon nor any arboreal processes can be directly credited, though multiple recursive draconic discontinuities impinge.