[Trekker4747]

Yeah, in the comic the turtles were taken from the lab by The Foot, Splinter attacks and then the turtles, Splinter and the mutagen all fall into the sewer. A cat runs off with one of the turtles in its mouth after it had been exposed to the mutagen. Later in the comic, after the turtles have grown to humanoid form, they mention their quest to find their "lost brother."