Cryptoplasm — ruling 1

You choose the target for the triggered ability when the ability is put onto the stack. You choose whether or not Cryptoplasm becomes a copy of that creature when the ability resolves.

Cryptoplasm — ruling 2

The copy effect lasts indefinitely. Often, it will last until it is overwritten by another copy effect (if it copies another creature on a future turn, perhaps.)

Cryptoplasm — ruling 3

If the creature is an illegal target when the ability tries to resolve, it won’t resolve. Cryptoplasm won’t become a copy of that creature; it remains whatever it was before.

Cryptoplasm — ruling 4

If another creature becomes a copy of Cryptoplasm, it will become a copy of whatever Cryptoplasm is currently copying (if anything), plus it will have Cryptoplasm’s triggered ability.

Cryptoplasm : règles, fonctionnement et interactions (MTG)

Cryptoplasm is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

At the beginning of your upkeep, you may have this creature become a copy of another target creature, except it has this ability.

How it works : Cryptoplasm

Original English text — Wizards of the Coast / Scryfall

#1June 1, 2011· Wizards of the Coast
You choose the target for the triggered ability when the ability is put onto the stack. You choose whether or not Cryptoplasm becomes a copy of that creature when the ability resolves.
No other card is named in this ruling.
#2June 1, 2011· Wizards of the Coast
The copy effect lasts indefinitely. Often, it will last until it is overwritten by another copy effect (if it copies another creature on a future turn, perhaps.)
No other card is named in this ruling.
#3June 1, 2011· Wizards of the Coast
If the creature is an illegal target when the ability tries to resolve, it won’t resolve. Cryptoplasm won’t become a copy of that creature; it remains whatever it was before.
No other card is named in this ruling.
#4June 1, 2011· Wizards of the Coast
If another creature becomes a copy of Cryptoplasm, it will become a copy of whatever Cryptoplasm is currently copying (if anything), plus it will have Cryptoplasm’s triggered ability.
No other card is named in this ruling.