e parts of its appended organs, liver, pancreas, &c., which are
concerned in delivering their secretions into the alimentary canal, as
well as the linings of those ramifying tubes in the lungs which convey
air to the places where gaseous exchange is effected. And from the
mesoblast originate the bones, the muscles, the heart and blood-vessels,
and the lymphatics, together with such parts of various internal organs
as are most remotely concerned with the outer world. Minor
qualifications being admitted, there remain the broad general facts,
that out of that part of the external layer which remains permanently
external, are developed all the structures which carry on intercourse
with the medium and its contents, active and passive; out of the
introverted part of this external layer, are developed the structures
which carry on intercourse with the quasi-external substances that are
taken into the interior--solid food, water, and air; while out of the
mesoblast are developed structures which have never had, from first to
last, any intercourse with the environment. Let us contemplate these
general facts.

Who would have imagined that the nervous system is a modified portion of
the primitive epidermis? In the absence of proofs furnished by the
concurrent testimony of embryologists during the last thirty or forty
years, who would have believed that the brain arises from an infolded
tract of the outer skin, which, sinking down beneath the surface,
becomes imbedded in other tissues and eventually surrounded by a bony
case? Yet the human nervous system in common with the nervous systems of
lower animals is thus originated. In the words of Mr. Balfour, early
embryological changes imply that--

"the functions of the central nervous system, which were originally
taken by the whole skin, became gradually concentrated in a special
part of the skin which was step by step removed from the surface,
and has finally become in the higher types a well-defined organ
imbedded in the subdermal tissues.... The embryological evidence
shows that the ganglion-cells of the central part of the nervous
system are originally derived from the simple undifferentiated
epithelial cells of the surface of the body."[58]

Less startling perhaps, though still startling enough, is the fact that
the eye is evolved out of a portion of the skin; and that while the
crystalline lens and its surroundings thus originate, the "