ter of fact, as the student will
perceive, frontal segment, parietal segment, and occipital segment,
can no longer be traced; and the mode of origin from trabeculae and
para-chordals show very clearly the falsity of this view. The vertebrate
cranium is entirely different in nature from vertebrae. The origin of the
parietals and frontals as paired bones in membrane reinforces this
conclusion.

Section 37. But as certainly as we have no such metameric
segmentation, as this older view implies, in the brain-case of the frog,
so quite as certainly is metameric segmentation evident in its
branchial arches. We have the four gill slits of the tadpole and their
bars repeating one another; the hyoid bar in front of these is evidently
of a similar nature; and that the ear drum is derived from an
imperforate gill slit is enforced by the presence of an open slit (the
spiracle) in the rays and dog-fish in an entirely equivalent position.
Does the mouth answer to a further pair of gill slits, and is the jaw
arch (palato-pterygoid + Meckel's cartilage) equivalent to the arches
that come behind it? This question has been asked, and answered in
the affirmative, by many morphologists, but not by any means by all.
The cranial nerves have a curious similarity of arrangement with regard
to the gill slits and the mouth; the fifth nerve forks over the mouth, the
seventh forks over the ear drum, the ninth, in the tadpole and fish,
forks over the first branchial slit, and the tenth is, as it were, a leash
of nerves, each forking over one of the remaining gill slits. But this
matter will be more intelligible when the student has worked over a
fish type, and need not detain us any further now.

Section 38. See also Section 13 again, in which is the suggestion
that the occipital part of the skull is possibly a fusion of vertebrae, a
new view with much in its favour, and obviously an entirely different
one from the old "segmental" view of the entire skull, discussed in
Section 36.

_Questions on the Frog_

[All these questions were actually set at London University
Examinations.] {In Both Editions.}

1. Give an account, with illustrative sketches, of the digestive organs
of the common frog, specifying particularly the different forms of
epithelium met with in the several regions thereof.

2. Describe the heart of a frog, and compare it with that of a fish and
of a mammal, mentioning in each case the great vessels which open
into