shell gland

In A the shell-gland and the mouth and the rudiment of the enteron are shown; primitive mesoderm cells.

The shell-gland is destined in Limnaeus to become very rapidly stretched out, and to disappear.

The shell-gland arises as an epiblastic thickening on the posterior and dorsal side.

In the centre is the mantle with the shell-gland which is now very considerably raised beyond the general surface.

Its cells first become differentiated into mesoblast and hypoblast after the shell-gland has become a fairly deep pit.