From the research notes of Gulistan, Master Biomancer:

A strange, filth-covered creature crawled up from the plumbing today. Our laboratory assistants noted a strange aura about it, which responded well to many forms of magic and was especially noted to accelerate the pace of evolution. Naturally, I had to summon one for myself.

Imagine my disappointment when the promised counter-doubling effect did not work as expected, and on the battlefield were myself, a 2/4 Master Biomancer, and a Corpsejack Menace with a measly two +1/+1 counters on it. A thorough vivisection of the creature (if such a thing can be called ‘alive’) led me to the answers I sought: unless a creature’s abilities specifically state otherwise, they only function when it is already on the battlefield. While a Master Biomancer’s ability is applying, it is while the creature entering is in that strange nowhere-land after it has already left wherever it came from (such as the stack) but before it has arrived onto the battlefield, so it’s too early for Corpsejack Menace’s ability to apply to the counters it gets from Master Biomancer.

Disheartened but not defeated, I subsequently tried summoning a Cloudfin Raptor. This creature entered with two counters–doubled–for a total of four. Because the Corpsejack Menace was already on the battlefield at this time, its counter-doubling effect was able to apply to the entering creature before it had fully materialized. Fascinating! Bear in mind that even though the word “placed” is found nowhere in the rules text of Master Biomancer’s ability, a creature “entering the battlefield with” counters already on it is considered to have had those counters “placed” on it.

Today’s Rules Tip written by Jen Wong