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The pore or opening leading into the cavity of invagination, or archenteron.

The archenteron gives off two lateral pounchs and thus becomes trilobed.

In the direct development Bateson showed that the three divisions of the coelom arise as pouches constricted off from the archenteron or primitive gut, thus resembling the development of the mesoblastic somites of Amphioxus.

We naturally assume, from what we have learnt, that the next stages will be the formation of a hollow blastosphere, invagination, a gastrula forming mesoblast by hollow outgrowths from the archenteron, and so on.

They distinguished in the middle layer two quite distinct elements, the mesoblast proper, formed by the evagination of the walls of the archenteron, and the mesenchyme, formed by free cells budded off from the germ-layers.