204 JIE. W. K. PAEKEE ON THE STEUCTHRE AND DEVELOPMENT 



PI. XXXVI. fig. 4, ex.br), appear outside each of the gill-bearing branchial arches, 

 four on each side, making in all nine pairs of these external cartilages. Three fourths 

 of the space between the superorbital bands is filled up by the gently convex cranial 

 roof, " tegmen cranii," which reaches as far as to the superoccipital (s.o), covering in 

 the whole of this flat-bottomed boat as with a slightly shelving deck. Between the tra- 

 beculae below and the superorbital above, there is a cartilaginous wall, reaching from the 

 ear-sac to the nasal sac ; this is the sphenoidal wall, and answers to the orbitosphenoid 

 and the alisphenoid of the higher vertebrata. A sectional view (PI. XLII. fig. 5, o.s) 

 shows how the cranial walls are chondrified, thickened above by the superorbital band, 

 and below by the trabeculae ; each of these regions of cartilage gives off a wing, the 

 upper wing roofing the eye-ball, and the lower forming a partial floor for it. 



The optic nerve (2) divides the orbital ala from the " ala major" — that is, regionally. 

 A primordial fissure between the alisphenoidal cartilage and the auditory sac (au) is 

 converted into a crescentic foramen, the " foramen ovale " (PI. XXXVI. fig. 5. 5). 

 Where the superorbital cartilage, grafted on to the nasal sacs, becomes 2^1'CBorMtal 

 (PL XXXVI. fig. 5), we get the meaning of the lower part of the ecto-ethmoid, or pars 

 plana, of human anatomy. The nasal branch of the ophthalmic nerve (5^) passes 

 between this downgrowth and the cranial wall. 



The hinder part of the fiattened cranial floor (PI. XXXVII. fig. 1, i v) is formed by 

 the investing mass, and is the notochordal region. It reaches from the foramen for the 

 internal carotid to the occipital condyles {oc.c). Sectional views, the one (PL XLII. fig. 6 j 

 in front, and the other (fig. 7) behind, show the degree of its downward convexity, and 

 its thickness at either end. The notochoid [nc) is still present in the hinder section ; it is 

 very small, and imbedded in the very substance of the cartilage ; its cartilaginous sheath 

 has coalesced with the lateral plates. The originally oval auditory sacs have taken the 

 form of their bulging contents, especially of the large semicircular canals, which are 

 very evident to sight on the upper surface (fig. 3, a.sc, h.sc,p.sc). 



The cartilage is complete both on the inner and o\xter sides, save where nerves and 

 vessels pass (PI. XLII. fig. 6) through the anterior part of the capsule and (fig. 7) 

 through the posterior part. The capsule has coalesced with the superorbital band in 

 fi-ont and above, with the superoccipital and tegmen cranii supero-mesially, and with 

 the investing mass below. The byomandibular is articulated below the horizontal 

 (external) semicircular canal. 



The changes that have taken place in the first postoral (mandibular and pterygoid) 

 are remarkable. The true apex, or metapterygoid (fig. 3, sp.c), is a fibrous band with a 

 grain of cartilage in the anterior lip of the first cleft, or " spiracle." The descending 

 bar sends forwards an enormous foot, the pterygo-quadrate bar ; and the descending part 

 is cut off from this, as the free mandible or articulo-Mecklian rod {q.pff, q, ar, mk). 

 Thus the secondary arch, or pterygo-quadrate, becomes practically as strong a bar in 

 front of the mouth as the lower part of the arch behind it. That nearly the whole 



