Adipose Offspring

Emerge {5}{W} (You may cast this spell by sacrificing a creature and paying the emerge cost reduced by that creature's mana value.)
When Adipose Offspring enters, create a 2/2 white Alien creature token. If Adipose Offspring's emerge cost was paid, instead create X of those tokens, where X is the sacrificed creature's toughness.

Doctor Who

A permanent's mana value is determined solely by the mana symbols printed in its upper right corner (unless that creature is the back face of a double-faced card, is a melded permanent, or is copying something else; see below). If the mana cost includes {X}, X is considered to be 0. If it's a single-faced card with no mana symbols in its upper right corner (because it's an animated land, for example), its mana value is 0. Ignore any alternative costs or additional costs (such as kicker) that were paid as the creature was cast.

Colored mana components of emerge costs can't be reduced with emerge.

For Adipose Offspring's last ability, use the toughness of the sacrificed creature as it existed when it was last on the battlefield, which may be different from its toughness in the graveyard.

Once you begin to cast a spell with emerge, no player may take actions until you're done. Notably, opponents can't try to remove the creature you wish to sacrifice.

The creature chosen to be sacrificed is still on the battlefield up through the time that you activate mana abilities. Its abilities may affect the spell's cost, be activated to generate mana, and so on. However, if it has an ability that triggers when a spell is cast, it will have been sacrificed before that ability can trigger.

The mana value of Adipose Offspring isn't affected by whether its emerge cost is paid. Its mana value is always 4.

The mana value of the back face of a transforming double-faced permanent is the mana value of its front face. The mana value of a melded permanent is the sum of the mana values of its front faces.

You may sacrifice a creature with a mana value of 0, such as a token creature that's not a copy of another permanent, to cast a spell for its emerge cost. You'll just pay the full emerge cost with no reduction.

You may sacrifice a creature with mana value greater than or equal to the emerge cost. If you do, you'll pay only the colored mana component of the emerge cost.