256 THE
ENZYME TREATMENT OF CANCER
of
the chick, how from an apparently undifferentiated mass—what we should now term
the “cleavage products”
—part
was gradually added to part, very much in the same way that one may any day
witness the building of a house from a heap of material—bricks, boards, beams,
windows, and what not—lying upon the ground beside the site. Study the
developmental history of an earthworm or leech, the whole development of a
skate or mammal, and the possibility of epigenesis may appear in another light.
But whether epigenesis be possible or not, nothing is more certain than that by
no modern observer has it ever been proved to be the mode of origin of an
embryo underlying the development. Incidentally, be it added, the foregoing
passage and the doctrine of epigenesis itself show how great is the importance
of “the embryo” in the embryological mind, and how little weight is really laid
upon other phenomena of the life-cycle prior to its appearance.
The
foundation-stones of epigenesis are direct development and a somatic origin of
germ-cells. If these be removed, the whole structure of epigenesis will crumble
away. Of the three tenets previously mentioned, possibly to none do
embryologists cling with greater tenacity than to direct development. In the
higher animals it is looked upon as the rule; indirect development is, indeed,
recognized under the term “metamorphosis,” though doubtless no advocate of
direct development has ever formed, or been able to formulate, any clear,
logical conception, free from metaphysics and “Naturphilosophie,” of what he
understands by “metamorphosis.” The term “metamorphosis” is not originally a
scientific one at all. It belongs to the realm of the supernatural, and it has
been imported into science—from mythology !—to account for facts, which at best
it scarcely succeeds in explaining away. To take an instance, the trochophore
larva of an annelid worm is said to become “metamorphosed” into the
annelid. Were this ever really to happen, the phenomenon would be exactly
comparable to that which in