DEVELOPMENT OE THE SKULL IN THE PIG. 
293 
are bent upon themselves, as the fingers in clutching ; hence the transverse crevice seen in 
the palate between the inner nares (fig. 4, i.n.c., tr.). This retral growth of the trabecular 
“ cornua ” is not so pronounced in the Frog (Phil. Trans. 1871 , Plate v.), but is equal to 
the Mammal in the Bird (“ Fowl’s Skull,” Phil. Trans. 1869, Plate lxxxi. figs. 1 & 2, tr.). 
The median part of the upper lip, which is transverse and quite rudimentary in the 
youngest embryo (fig. 3. u.l .), has developed in a somewhat older specimen (fig. 4, pn.) 
into a pointed retral flap. This flap hides an azygous projection of the trabecular com- 
missure, the “prenasal cartilage;” this axis of the premaxillaries is a part largely developed 
in Birds (see “ Fowl’s Skull,” Plates lxxxi.-iii. pn.), where it is first retral, then vertical, 
and then foreturned, so that it is the principal factor in the exaggerated prognathism of 
that Class. Outside this process the trabecular cornua are at present clubbed and bulbous 
(Plate XXVIII. figs. 3, 4,&5,c.£r.) ; afterwards they each send backwards a recurrent rod*. 
The general appearance of the trabeculae, as seen from above, is shown in Plate XXIX. 
fig. 4 ; their varying thickness is displayed in sections (Plate XXIX. figs. 1, 2, 3, & 5, tr.). 
Second Preoral Arch. — Even in the Osseous Fish I found the pterygo-palatine arch 
both late and feeble in its development ; in the Frog it is a long time before it appears, 
and grows very slowly, and is never more than a long conjugational band between the 
trabecular and mandibular rod. In the Mammal, as in the Bird, this primarily feeble 
rod is ossified hurriedly, as it were, before the cells can acquire any intermediate sub- 
stance (see “Fowl’s Skull,” Plate lxxxi. figs. 1, 6, & 11); yet in the present instance 
the bony plates that arise in and around these small sigmoid granular rods are some of 
the most complicated and the most massive in the whole head and face. Even through 
the palatal skin the hooked tops of the preoral arches can be seen (fig. 4) ; but whilst 
those of the trabeculse grow inwards, those of the pterygo-palatine bars grow upwards 
and outwards, persistent in the “ hamular process.” The direction of the whole bar 
(Plate XXVIII. figs. 4 & 5, ppg.) is downwards and forwards, and their extremities or 
“ cornua” approach each other below the trabeculae : they are at present far apart in 
this originally cleft palate (figs. 4 & 5) ; the fold of mucous membrane covering each on 
its inner side gradually grows towards its fellow, and they eventually meet and coalesce. 
The thick cushion outside each bar is the nidus in which the maxillary and malar are 
developed; and the whole maxillo-palatine mass is a mere process or outgrowth of 
the first (postoral) arch, and is not an independent morphological region. At present the 
arch is subocular-, but it does not correspond to the subocular bar of the Tadpole 
(“ Frog’s Skull,” Plate v.), which is formed by the extremely long pier of the mandibular 
arch, the arrested conjugational pterygo-palatine lying quite in front of the eyeball. 
* The distinctness of these rods from the surrounding tissues is purposely exaggerated in the accompanying 
illustrations, for they are imbedded in a gelatinous tissue rich with enclosed granules or young cells, whose 
protoplasmic substance takes up the carmine very freely ; and the differentiation of these rods is at present a 
matter of degree, that part of the blastema which will become hyaline cartilage being the most compact and 
crowded with young cells ; next to this the nascent perichondi’ium ; and the most gelatinous part outside is the 
rudimentary condition of the loose stroma or areolar connective tissue. 
