The blastopore, together with the whole embryo, now elongates.
The hypocotyl usually elongates, by its cells increasing very greatly in the longitudinal direction both in number and size, so that the cotyledons are raised into the air as the first foliage-leaves.
As growth proceeds, one hemisphere remains relatively small, the other elongates and enlarges.
The ectoderm behind the ciliated ring develops spicules, and the post-oral region of the larva elongates.
In Teredo a similar tube is formed as the animal elongates (boring in wood), the original shell-valves not adhering to it but remaining movable and provided with a special muscular apparatus in place of a ligament.
