substituting for the "Archetype" of pure
morphology what one may perhaps best call the _embryological archetype_.
How the transition was made we can best see by following out the course
of discovery in one particular line. We choose for this purpose the
development of the skull, a subject which excited much interest at this
time and upon which much quite fundamental work was done, particularly
by Rathke and Reichert.

Following up his discovery of gill-slits and arches in the embryos of
birds and mammals, Rathke in two papers of 1832[199] and 1833[200] worked
out the detailed homologies of the gill-arches in the higher
Vertebrates. He describes how in the embryo of the Blenny there is a
short, thick arch between the first gill-slit and the mouth. A furrow
appears down the middle of the arch dividing it incompletely into two.
In the anterior halves a cartilaginous rod is developed which is
connected with the skull; these rods become on either side the lower jaw
and "quadrate." In the posterior halves two similar rods are formed
which develop into the hyoid. The hyoid is at first connected with the
skull, but afterwards frees itself and becomes slung to the "quadrate."
From the hinder edge of the hyoid arch grows out the membranous
operculum, in which develop later the opercular bones and branchiostegal
rays. The upper jaw is an independent outgrowth of the serous layer.

The serial homology of the lower jaw and quadrate with the hyoid and
with the true gill-arches was thus established in fish, and Rathke had
little difficulty in demonstrating a similar origin of lower jaw and
hyoid in the embryos of higher Vertebrates. He could even, as we have
noted before, find the homologue of the operculum in a flap which grows
out from the hyoid arch in the embryo of birds.

But Rathke could not altogether shake himself free from the
transcendental notion of the homology of jaws with ribs, and this led
him to draw a certain distinction between the first two and the
remaining gill-arches, by which the homology of the former with the ribs
was asserted and the homology of the latter denied. He thought he could
show that the skeletal structures (lower jaw, "quadrate," and hyoid) of
the first two arches were formed in the serous layer, just like true
ribs, and like them in close connection with the vertebral skeletal
axis. The other, "true," gill-arches appeared to him to be formed in the
mucous layer, in the lining of the alimentary c