f their separate parts. This idea must first be worked out,
though possibly with modifications, before more special ideas can find
play. The result of the latter process, however, is that what was formed
by the first idea is not so much hidden as partially or wholly
destroyed" (p. 135).

Rathke's general paper on the development of the skull in Vertebrates[211]
treats the matter on a broader comparative basis than his paper on the
adder, and takes into account all the vertebrate classes, in so far as
their development was then known. He here makes the interesting
suggestion, later entirely confirmed, that the _basis cranii_ or basilar
plate is first laid down as two strips, one on each side of the
chorda--the structures now known as parachordals (pp. 6, 27). For this
supposition, he thinks, speaks the structure of the skull in
_Ammocoetes_, which in this respect is the simplest of all Vertebrates
(pp. 6, 22). In _Ammocoetes_, as Johannes Mueller had shown, the
foundation of the skull is formed by two long cartilaginous bars,
between the hinder portions of which the notochord ends. In these Rathke
was inclined to see the homologues of his trabeculae, and of the
parachordals which he was ready to assume from his embryological
observations.

Mueller was, of course, very ready to accept Rathke's opinions on this
subject, for he considered that they supported his own theory of the
vertebral nature of the skull. After describing in his _Handbuch der
Physiologie_ the cartilaginous bands in _Ammocoetes_ and their highly
differentiated homologues in the Myxinoids, he writes in the later
editions, "Hence we see that in the cranium, as in the spinal column,
there are at first developed at the sides of the chorda dorsalis two
symmetrical elements, which subsequently coalesce, and may wholly
enclose the chorda. Rathke has recently observed, in the embryos of
serpents and other animals, before the formation of the proper cranial
vertebrae, two symmetrical bands of cartilage, similar to those which I
discovered as a persistent structure in _Ammocoetes_.... At a later
period the _basis cranii_ of vertebrate animals contains three parts
analogous to the bodies of vertebrae, the most anterior of which, in the
majority of animals, is generally small, and its development frequently
abortive, whilst in man and mammiferous animals the three are very
distinct. These parts are developed by the formation of three distinct
points of ossificati