292 



ME, W. K. PAEKEE ON THE STEUCTUEE AND 



Where the facial arches most closely imitate ribs, as in the first and second postoral, 

 the " capitulum " and the " tuberculum " are applied to a part to which they have no 

 proper morphological relation — namely, to a sense-capsule. This is one of the many 

 modifications the morphological elements are subjected to in the cephalic region. 



The original pattern of the facial system of a vertebrate is simple in the extreme ; the 

 paired rods are accurately like each other, but their development is not quite synchro- 

 nous ; the secondary preoral pterygo-palatine {p-pg.) is overshadowed and slow in growth 

 (see Plate XXVIII. fig. 2, where the arches are drawn as though the object were trans- 

 parent) *. The facial thickenings between the clefts which contain the arches may be seen 

 with considerable clearness, especially in front (fig. 3); here in front of (above) the mouth 

 towards the mid line we see the clubbed ends of the trabeculse [tr.) roofed over by the 

 nasal sacs. Below and somewhat behind these are the pterygo-palatine arches (p.pg.), 

 in the thick outer wall of which the maxillaries and malars will be developed, and the 

 pith of which will become, by early ossification, the palatine and pterygoid bones. 

 Below the inferior, transverse, large mouth, the thickenings which contain the first and 

 second postorals are seen — Meckelian and hyoid. The cleft which is formed between 

 the trabecular and the pterygo-palatine bars is best seen in the side view (figs. 1 & 2, cl.l) ; 

 it opens in the inner canthus of the eye. The two pairs of preoral rods will be best 

 understood by reference to a palatal view of the skull with the postorals cut away (fig. 4), 

 and to the diagrammatic view of the skull and face as seen from below (fig. 5). It is 

 easy to see, by a reference to the palatal view (fig. 4), that we are now standing on the 

 same level as the " Dipnoi " amongst the Fishes ; the external nostril (e.n.) and the 

 internal (i.n.) lie on the same plane ; a free intervening growth of cartilage, binding 

 the arches together, with no further metamorphosis of the parts, would produce a true 

 parallel to the skull of those remarkable Fish. The sinuosities of the u])turned palate 

 (fig. 4), its plaits and its crevices, are easily understood by reference to the diagram 

 (fig. 6). 



First Preoral Arch. — The trabecular rods form together an elegantly lyriform struc- 

 ture ; they already have begun their extensive " commissure," being parallel now in 

 their fore half Behind, they arc like callipers, and the blades are at some distance from 

 each other ; their apices, sharpened off, seem to approach the fore end of the investing 

 mass (i.v.) ; but a sectional view (fig. 6, tr.cm., i.v.) corrects this error, and shows that these 

 diverse parts lie on a totally distinct plane and far from each other, a fact I pointed out 

 long ago in my paper on the Frog (Phil. Trans. 1871, Plate iii, p. 143). These trabe- 

 cular blades embrace the pituitary body (p?/.) ; but tlicir curve does not conform to 

 its shape, and is altogether independent of it, being the proper " habit " or morpholo- 

 gical yW^'o/i of the arch. After forming the elegant, ])yriform, primordial pituitary 

 space, the trabeculae become thicker, narrower, and Vis closely side by side ; this is soon 

 followed by fusion of their edges — the formation of th(> trabecular commissure (see 

 Plate XXIX. fig. 4, tr.cm.). These two rods do not end as a straight bar, but in iVont 



* In my paper on the Frog (Phil. Trans. 1871, p. 148) the pterygo-palatine arcade is described as a secondary 

 structure ; in tliat on the Salmon {ihkl. 1873, p. 109) it is spoken of as independent. It is a secondary arch. 



