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, etc.), 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-otic 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 trabeculae are 

 prechordal in position, ])eing situated beneath the twixt-brain 

 and cerebral hemispheres, and extending forward through the 



