. 11.--Embryonic Cranium of the Adder. Ventral Aspect.
(After Rathke.)]

During the formation of the ear-capsule the cranial basis develops from
a plate to a trench, for in its hinder section the side parts grow up to
form the side walls of the brain, in exactly the same way as the
processes of the vertebral rudiments grow up to enclose the spinal
column (pp. 122, 192). The foundations of the skull are now complete,
and ossification gradually sets in. The basioccipital is formed
in the posterior part of the _basis cranii_, and the exoccipitals in the
side walls of the trench in continuity with the fundament of the
basioccipital (see Fig. 11). The supraoccipital is formed in cartilage
above the exoccipitals. The basisphenoid develops, like the
basioccipital, in the flat _basis cranii_, but towards its anterior
edge, between the large foramen (_h_) and the pituitary space (_i_). It
is formed from two centres, each of which is originally a ring round the
carotid foramen. The presphenoid develops in isolation between the
lateral trabeculae, just behind the point where they fuse. The side parts
of the basisphenoid and presphenoid (forming the alisphenoids and the
orbitosphenoids respectively) develop in cartilage separately from the
cranial basis, not like the exoccipitals in continuity with it. The
hinder parts of the trabeculae become enclosed by two processes of the
basisphenoid; their front parts remain in a vestigial and cartilaginous
state alongside the presphenoid. The frontals and parietals show a
peculiar mode of origin in the adder, differing from their origin in
other Vertebrates. The frontals develop in continuity with the
orbitosphenoids, the parietals in continuity with the alisphenoids, and
so have much resemblance with the vertebral neural arches which surround
the spinal column (p. 195).

Through Rathke's work the real embryonic archetype of the vertebrate
skull was for the first time disclosed. Rathke discussed this archetype
and its relation to the vertebral theory of the skull in another paper
of the same year (1839), but before going on to this paper, we shall
quote from the paper on the adder the following passage, remarkable for
the clear way in which the idea of the embryological archetype is
expressed. "Whatever differences may appear in the development of
Vertebrates, there yet exists for the different classes and orders a
universally valid idea (plan, schema, or type) ruling the first
formation o