appearance on either side of it; these prove to be
the rudiments of the vertebrae--or separate bones of the backbone--and
gradually close round the cord. The heart is at first merely a
spindle-shaped enlargement of the main ventral blood-vessel. The nose is
at first only a pair of depressions in the skin above the mouth.

When the human embryo is only a quarter of an inch in length, it has
gill-clefts and gill-arches in the throat like a fish, and no limbs. The
heart--as yet with only the simple two-chambered structure of a fish's
heart--is up in the throat--as in the fish--and the principal arteries
run to the gill-slits. These structures never have any utility in man or
the other land-animals, though the embryo always has them for a time.
They point clearly to a fish ancestor.

Later, they break up, the limbs sprout out like blunt fins at the sides,
and the long tail begins to decrease. By the twelfth week the human
frame is perfectly formed, though less than two inches long. Last of
all, it retains its resemblance to the ape. In the embryonic apparatus,
too, man closely resembles the higher ape.

_III.--Our Ancestral Tree_

The series of forms which we thus trace in man's embryonic development
corresponds to the ancestral series which we would assign to man on the
evidence of palaeontology and comparative anatomy. At one time, the
tracing of this ancestral series encountered a very serious check. When
we examined the groups of living animals, we found none that illustrated
or explained the passage from the non-backboned--invertebrate--to the
backboned--vertebrate--animals. This gap was filled some years ago by
the discovery of the lancelet--_Amphioxus_--and the young of the
sea-squirt--_Ascidia_. The lancelet has a slender rod of cartilage along
its back, and corresponds very closely with the ideal I have sketched of
our primitive backboned ancestor. It may be an offshoot from the same
group. The sea-squirt further illustrates the origin of the backbone,
since it has a similar rod of cartilage in its youth, and loses it, by
degeneration, in its maturity.

In this way the chief difficulty was overcome, and it was possible to
sketch the probable series of our ancestors. It must be well understood
that not only is the whole series conjectural, but no living animal must
be regarded as an ancestral form. The parental types have long been
extinct, and we may, at the most, use very conservative living types to
illus