| protostoma | During gastrulation cells on the surface of the embryo move into the interior to form the mesoderm and endoderm. The opening formed by this invagination of cells is the blastopore. It is an opening from the archenteron, the primitive gut, to the exterior. In some animals this opening becomes the anus, whilst in others it closes up and the anus opens at the same spot or nearby. In some animals, for example chick, invagination occurs without a true blastopore and the site at which the cells move in (primitive streak) may be termed a virtual blastopore. (18 Nov 1997) |
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