226 



GENERAL ORNITHOLOGY 



PART 11 



the ceratohyal and hasihyal bones ; the pieces cbr, ebr, and bhr are in 

 the third arch, and form respectively the ceratobranehial, epibranchial, 

 and hasibratichial bones. These pieces of the third arch have already- 

 outgrown those of the second arch, and they will form the greatest 

 part of the hyoid bone. 



In the second stage, after the fifth day of incubation, but before 

 any ossification has begun, a vertical section shows the appearances 

 represented in Fig. 66. The parachordal and trabecular cartilages 



are applied to each 

 other unconformably, 

 the latter rising high 

 between second and 

 third cerebral vesicles 

 to form the posterior 

 pituitary wall, pel, in 

 which the axial skele- 

 ton properly ends. 

 There are other 

 changes in the para- 

 chordal cartilages. 

 The internasal plate, 

 formed by the union 

 of the trabeculse in 

 front of the pituitary 

 space, has become a 



Fig. 66. — Head of a chick, second stage, after five days of j.- i j- n 



incubation, section in profile ; X 6 diameters. CTl, ot2, cj>3, first, vertical median Wall 

 second, and third cerebral vesicles ; 1, place of the first nerve, 

 the olfactory ; 2, place of second nerve, the optic ; 



-c.v,!l 



<It!l 



between the olfactory 



.... , ^ , ic, internal 



carotid artery, running into skull at what was originally the and OPtic chambers 

 pituitary space, now an opening bounded in front by the an- ^ 



terior, ad, behind by the posterior, pel, clinoid walls ; nc, noto- 

 chord ; oo, occipital condyle, thence to pd being the original 

 parachordal cartilage, here seen in profile ; eo, exoccipital ; dh, 

 ethmoid, with ps, its presphenoid region posteriorly, and jm, 

 prenasal part ; this whole plate afterward developing into parts 

 of the nose and the partition between the eyes ; pa, palatine ; 

 pg, pterygoid region ; pa and pg reference lines are in the 

 chick's mouth ; mk, Meckelian cartilage (lower jaw) ; cA and bft, 

 ceratohyal and basihyal parts of the hyoid or tongue bone. 

 {After Parker.) 



of the right and left 

 sides (pn and eth, to 

 ps and ale). This 

 partition, besides 

 forming finally the 

 interorhital septum 



which divides the 

 right and left orbits, will undergo further notable changes 

 in direction, and will develop lateral plates and processes, which 

 will make up the nasal labyrinth and the partition between the 

 cavity of the nose and that of the eye, when any exists. Such 

 lateral developments of the ethmoid plate are the alietlmoid, aliseptal, 

 and alinasal. This plate extends backward in mid-line to the optic 

 foramen, 2, ending in the anterior clinoid wall, act, separated from the 

 (parachordal) posterior clinoid, pel, wall by the original pituitary 

 space, now the opening through which the carotid arteries, ic, enter 

 the brain cavity. Besides ethmoidal parts proper, the plate 



