cells and forms its lining.  This cup-shaped embryo is called the gastrula.  The cup deepens somewhat and becomes ovoid.  Take a boiled egg, make a hole in the smaller end and remove the yolk, and you have a passable model of a gastrula.  The shell corresponds to the ectoderm or outer layer of smaller cells; the layer of “white” represents the entoderm or lining of larger cells.  The space occupied by the yolk corresponds to the archenteron or primitive digestive cavity; and the opening at the end to the primitive mouth or blastopore.  Ectoderm and entoderm unite around the mouth.  Both the blastosphere and gastrula often swim freely by flagella.

You can hardly have failed to notice how closely the gastrula corresponds to a hydra, and many facts lead us to believe that the still earlier ancestor of the hydra was free swimming, and that the tentacles are a later development correlated with its adult sessile life.  Yet we must not forget that the hydra is even now not quite sessile, it moves somewhat.  And our ancestor was almost certainly a free swimming gastraea, or hypothetical form corresponding in form and structure to the gastrula.  The ancestor of man never settled down lazily into a sessile life.

But how is an adult worm or vertebrate formed out of such a gastrula?  To answer this would require a course of lectures on embryology.  But certain changes interest us.  Between the ectoderm and entoderm of the gastrula, in the space occupied by the supporting membrane of hydra, a new layer of cells, the mesoderm, appears.  This has been produced by the rapid growth and reproduction of certain cells of the entoderm which have migrated, so to speak, into this new position.  In higher forms it becomes of continually greater importance, until finally nearly all the organs of the body develop from it.  In our bodies only the lining of the mid-intestine and of its glands has arisen from the entoderm.  And only the epidermis, or outer layer of our skin, and the nervous system and parts of our sense-organs have arisen from the ectoderm.  But our mid-intestine is still the greatly elongated archenteron of the gastrula.

We may therefore compare the hydra or gastrula to a little portion of the lining of the human mid-intestine covered with a little flake of epidermis.  This much the hydra has attained.  But our bones and muscles and blood-vessels all come from the mesoderm by folding, plaiting, and channelling, and division of labor resulting in differentiation of structure.  Of all true mesodermal structures the hydra has actually none, but in the ectodermal and entodermal cells he has the potentiality of them all.  We must now try to discover how these potentialities became actualities in higher forms.