Sea urchin gastrulation.
<P>
   Invagination of the vegetal plate in the single-layered blastula forms
   the blastopore, which will become the anus.
<P>
   Further  differentiation yields a gastrula with three embryonic tissue
   layers.
<P>
<BR><LI> Endoderm.  Infolding  forms  the archenteron, which later develops
into  the  digestive  tube  with  a  second opening that becomes a
mouth.
<BR><LI> Mesoderm.  Mesenchyme  cells migrate into the blastocoel, and will
later form the skeleton.
<LI> Ectoderm.