pment of the frog's
skull with the persistent conditions of the skull in the _Amphioxus_,
the lamprey, and the shark, we shall discover the model and type of the
latter in the former. The skull of the _Amphioxus_ presents a
modification of that plan which is exhibited by the frog's skull when
its walls are still membranous and the notochord is not yet embedded in
cartilage. The skull of the lamprey is readily reducible to the same
plan of structure as that which is exhibited by the tadpole when its
gills are still external and its blood colourless. And finally, the
skull of the shark is at once intelligible when we have studied the
cranium in further advanced larvae, or its cartilaginous basis in the
adult frog" (p. 577). Development, therefore, proves what comparative
anatomy could only foreshadow--the unity of plan of all vertebrate
skulls, ossified and unossified alike. "We have thus attained to a
theory or general expression of the laws of structure of the skull. All
vertebrate skulls are originally alike; in all (save _Amphioxus_?) the
base of the primitive cranium undergoes the mesocephalic flexure, behind
which the notochord terminates, while immediately in front of it the
pituitary body is developed;[221] in all, the cartilaginous cranium has
primarily the same structure--a basal plate enveloping the end of the
notochord and sending forth three processes, of which one is short and
median, while the other two, the lateral trabeculae, pass on each side of
the space on which the pituitary body rests, and unite in front of it;
in all, the mandibular arch is primarily attached behind the level of
the pituitary space, and the auditory capsules are enveloped by a
cartilaginous mass, continuous with the basal plate between them. The
amount of further development to which the primary skull may attain
varies, and no distinct ossifications at all may take place in it; but
when such ossification does occur, the same bones are developed in
similar relations to the primitive cartilaginous skull" (p. 578).

In a word, there is a general plan or primordial type which is
manifested in the higher forms most clearly in their earliest
development--an embryological archetype therefore.

Huxley now goes on to consider the relation of this general plan or type
of the skull to the structure and development of the vertebral column.
Does the skull in its development show any signs of a composition out of
several vertebrae? The vertebral