pays much attention to development.
He describes the formation of the vertebrae in elasmobranch embryos; for
the facts regarding other Vertebrates he relies largely on work by
Rathke (_Blennius_, 1833) and Duges (1834). He recognises as the basis
of his comparisons the homology of the notochord in all vertebrate
embryos with the persistent notochord which forms the chief part or the
whole of the vertebral column in the Cyclostomes. The notochord
possesses an inner and an outer sheath and the outer sheath is
continuous with the _basis cranii_ (p. 92). It is in the outer sheath
that the vertebrae develop--from four separate pieces, in fish at least,
plus an additional element which helps to form the centrum. The skull of
Vertebrates consists, according to Mueller, of three vertebrae, whose
centra are the basioccipital, the basisphenoid and the presphenoid.
Other bones besides those belonging to the vertebrae are present, but
this formation out of three vertebrae gives the essential schema for the
skull. Now the brain capsule, like the sheath of the spinal cord, is a
development from the outer sheath of the notochord. If the skull
consists of vertebrae we should expect the centra of the skull-vertebrae
to develop in the outer sheath at the sides of the cranial section of
the notochord as two separate halves, just as do the bodies of the
vertebrae; we should expect further the cartilaginous side-walls of the
cranium to develop in the membranous brain-sheath just as the neural
arches develop in the membranous sheath of the spinal column. In
Rathke's discovery (!) of a segmentation of the _basis cranii_ into
three parts, and of the isolated formation of the vomer, Mueller sees a
confirmation of his view that the skull is composed of three and not
four vertebrae. But there is nothing in Rathke's observations to support
the idea that the centra of the cranial vertebrae are formed from
separate halves. Mueller has to be content with a reference to the state
of things in _Ammocoetes_ (which, by the way, he did not know to be the
young of _Petromyzon_). In the simple skull of _Ammocoetes_ the base is
formed chiefly by two cartilaginous bars lying more or less parallel
with the longitudinal axis of the skull and embracing with their hinder
ends the cranial portion of the notochord.

These bars, declares Mueller, are clearly the still separate halves of
the _pars basilaris cranii_, and represent the divided centra of the two
hinder