ich is parallel with the
head. This upper segment forms the foundation for the bones of the
upper jaw. The lowest segment of the cartilaginous rod becomes
Meckel's cartilage, and on the outer side of this the bones of the
lower jaw are formed. The middle segment becomes in mammals the incus
(one of the ear-ossicles), and in birds the quadrate. Meckel's
cartilage, which was discovered by Meckel[205] in fish, amphibians and
birds, is a long strip of cartilage which runs from the ear-ossicle
known as the hammer in mammals,[206] to the inside of the mandible.
Reichert shows how this relation comes about. The hammer, according to
his observations on the embryo of the pig, is simply the proximal end
of Meckel's cartilage, which later becomes separated off from the long
distal portion (see Fig. 9). The third ear-ossicle of mammals, the
stapes, comes not from the first arch but from the second. The
cartilaginous rod of the second arch segments like the first into
three pieces. Of these the uppermost disappears, the middle one, which
lies close up to the labyrinth of the ear, becomes the stapes, and the
lowest becomes the anterior horn of the hyoid. The stapes forms a
close connection with the hammer and the incus. In birds, where there
is a single ear-ossicle, the columella, the middle piece of arch I
forms, as we have seen, the quadrate, by means of which the lower jaw
is joined to the skull. The proximal end of Meckel's cartilage, which
in mammals forms the hammer, here gives the articular surface between
the lower jaw and the quadrate. The columella is formed from the
middle piece of the three into which the cartilage of the second arch
segments. It is, therefore, the homologue of the stapes in mammals.
The third arch takes a varying share, together with the second, in the
formation of the hyoid apparatus.

[Illustration: FIG. 9.--Meckel's Cartilage and Ear-ossicles in Embryo
of Pig. (After Reichert.)]

In this paper Reichert made a distinct advance on the previous workers
in the same field--Rathke, Huschke, von Baer, Martin St Ange, Duges.
Huschke was indeed the first to suggest that both upper and lower jaws
were formed in the first gill-arch. But both von Baer and Rathke[207] held
that the upper jaw developed as a special process independent of the
lower jaw rudiment, and the actual proof that the upper jaw is a
derivative of the first visceral arch seems to have been first supplied
by Reichert. His brilliant work on th