own
as the "parachordals" and "trabeculae," of sense capsules enclosing the
nose and ear, and of various roofing bones. In the historical
development of the skull three grades become apparent; a primitive
stage, as seen in Amphioxus, where there is nothing but a fibrous
investment of the nervous structures; a cartilaginous grade, as seen
in the skate or shark, where the skull is formed of cartilage, very
imperfectly hardened by earthy deposits; a bony stage, seen in most of
the higher animals. He shewed that in actual development of the higher
animals these historical grades are repeated, the skull being at first
a mere membranous or fibrous investment of the developing nervous
masses, then becoming cartilaginous, and, lastly, bony. He made some
important prophetic remarks as to the probable importance that future
embryological work would give to the distinction between cartilage and
membrane bones--a prophecy that has been more than fully realised by
the investigations of Hertwig and of others. Our present knowledge of
the skull differs from Huxley's conception practically only in a
fuller knowledge of details. We know now that throughout the series
there is a primitive set of structures common to all animals higher in
the scale than Amphioxus, and forming the base and lateral walls of
the skull. This is termed the Chondrocraninm, because it is laid down
in cartilage; it is composed of the separate elements which Huxley
indicated, and, in different animals, as Huxley suggested, the exact
limits of the ossification of the primitive cartilages differ in
extent, but occur in homologous situations. This primitive skull is
roofed over by a series of membrane bones which have no connection in
origin with the other portions of the skull, and which have no
representative in the vertebral column, but which are the direct
descendants of the bony scales clothing the external skin in
cartilaginous fishes. In one respect only was Huxley erroneous. Partly
by inadvertence, and partly because the minute details of vertebrate
embryology became really familiar to zooelogists only after the
elaborate work of Balfour of Cambridge, Huxley, in his account of the
formation of the first beginnings of the skeleton in the embryo, made
confusion between the walls of the primitive groove, which, in
reality, give rise to the nervous structures, and those embryonic
tissues which form the skeletal system.

The next great piece of work which we may