Do hatchlings grow only lengthwise?
No, turtles grow all over, sideways, lengthways, and in weight. When they are young they grow fairly quickly, and can reach 30-40 cm in length in one year. As they grow older, much like in the case with humans, their growth rates slow down, to a point that they may only grow by 1 or 2 mm, or not grow at all, by the time they are mature and return to nest.
