GENERA L OKNITHOL OGY 



third post-oral arch, Ir 1 and hr 2, which retain the tongue-bone in 

 position, without however articulating it with the skull. The hand 

 of the trowel of cartilage soon segments itself off from the ear- 

 capsule, bringing away with it a small oval piece of the periotic wall, 

 which piece is the true stapes, and the oval space in which it fits is 

 the fenestra ovalis leading into the inmost ear (the cochlea). The 

 broad part of the trowel-blade is the extrastapedial part, on which 

 the membrana tympani, or ear-drum, will be stretched. The stylo- 

 hyal, stk, will join the extrastapedial plate, and the afterward 



chondrified band of union will 

 be the infrastapeclial, ist. (Figs. 

 71, st, and 83.) 



Eeturning now to the chick's 

 head, which we left to examine 

 the intricate ear-parts at the 

 proximal end of the second 

 post-oral arch, we see by Fig. 

 6S how rapidly the parts are 

 shaping themselves at the end 

 of this second stage of develop- 

 ment. This figure shows the 

 skull, in which 

 no trace of ossification has 

 appeared, excepting in the 

 under mandible. The brain 



h. w!--*^ 



Fig. OS. — Skull of chick, second stage, in profile, 

 brain and membranes removed to show cartilagi- 

 nous formations, x 4 diameters, eth, ethmoid, 

 forming median nose-parts and interorbital sep- 

 tum ; developing lateral parts, as ale, aliethmoid, 

 als, aliseptum, aln, alinasal, pp, partition between cartilaginous 

 nose and eye ; pn, pre-nasal cartilage ; ps, pre- '^ 



sphenoidal part of mid-ethmoid ; 2, optic foramen ; 

 as, alisphenoid, walling brain-box in front ; pj, 

 l)ost-frontal, bounding orbit behind ; pa, pg, pala- 

 tine and pterygoid ; q, quadrate ; so, supra- 

 occipital ; eo, exoccipital ; oc, occipital condyle, 



borneuponbasioccipital, and showings, remains and membraUOUS parts of the 

 or Tiotop.norfl ! these of.p.niirn.ls nonnrl flip fArnmpn _ * 



cranium have been removed. 

 The roof of the skull never 



becomes cartilaginous, bone 



of notochord ; these occipitals bound the foramen 

 magnum, and eo expands laterally to form a tym- 

 panic wing, circumscribing the external auditory 

 orifice behind and below ; fe', j)sc,' horizontal and 

 posterior vertical semicircular canals of ear ; fr, 

 st, fenestra rotunda and fenestra ovalis, leading 

 into inner ear, latter closed by foot of the stapes ; there growiug directlv f rom 

 ink, ch, hh, hbr, cbr, ehr, -parts of jaw and tongue, ,1 ■, t ,^ i i 



as named in Figs. 65, 66, and 67. (After Parker.) the memOTaue ; and the whole 



of the chondro - cranium, as 

 shown in the figure, is one continuous cartilaginous structure (like the 

 whole skull of an adult shark or skate), excepting the parts of the post- 

 oral arches, which are separate. The auditory capsule is environed 

 by occipital cartilage, eo, stretching over the back of the skull, and 

 by wing-like growths (alisphenoids, as) which wall most of the brain- 

 box in front. The high orbito-nasal septum is a continuous vertical 

 plate of cartilage, upgrowing from the tract of the conjoined tra- 

 becular. Lateral developments of this ethmoidal wall, in front, are 

 divided into several recognisable parts, ale, als, aln, the latter being 

 the external nostril ; pjj is a transverse partition between the orbital 

 and nasal chambers. The nasal cartilages ultimately become much 

 convoluted to form the nasal labyrinth, among the convolutions of 

 which Avill be the superior and inferior turbinal cartilages, in addi- 



