428 THE DEVELOPMENT OF THE CHICK 
Development of the Cartilaginous or Primordial Cranium. 
(1) The Neurocranium. The neurocranium is derived from the 
mesenchyme of the head, the origin of which has been described 
previously. The mesenchyme gradually increases in amount and 
forms a complete investment for the internal organs of the head. 
It is not all destined, however, to take part in the formation of 
the skeleton, for the most external portion forms the derma and 
subdermal tissue; and, internal to the skeletogenous layer, the 
membranes of the brain and of the auditory labyrinth, etc., are 
formed from the same mesenchyme. 
The notochord extends forward in the head to the hypophysis 
(Figs. 67, 88, ete.), and furnishes a basis for division of the 
neurocranium into chordal and prechordal regions. Within the 
chordal division again, we may distinguish pre-otic, otic, and 
post-otie regions according as they are placed in front of, around, 
or behind the auditory sac. The part of the postotic region 
behind the vagus nerve is the only part of the neurocranium 
that is primarily segmental in origin. The sclerotomes of the 
first four somites (Figs. 63 and 117) form this part of the skull; 
and at least three neural arches, homodynamous with the verte- 
bral arches, are formed in an early stage, but fuse together while 
still membranous, leaving only the two pairs of foramina of the 
twelfth cranial nerve as evidence of the former segmentation. It 
is also stated that membranous costal processes are found in 
connection with these arches, but they soon disappear without 
chondrifying. 
The primordial neurocranium is performed in cartilage and 
corresponds morphologically to the cranium of cartilaginous 
fishes. However, it never forms a complete investment of the 
brain; except in the region of the tectum synoticum it is wide 
open dorsally and laterally. It is subsequently replaced by 
bone to a very great extent, and is completed and_ reinforced 
by numerous membrane bones. 
The neurocranium takes its origin from two quite distinct 
primordia situated below the brain, viz., the parachordals and 
the trabeculae. The former develop on each side of and around 
the notochord, being situated, therefore, behind the cranial 
flexure and beneath the mid- and hind-brain; the trabecule are 
prechordal in position, being situated beneath the twixt-brain 
and cerebral hemispheres, and extending forward through the 
