DEVELOPMENT OF THE SKULL IN THE PIG. 



293 



are bent upon themselves, as the fingers in clutching ; hertte 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 Lxxxr. figs. 1 & 2, tr.). 

 The median part of the upper lip, which is transverse and quite rudimentary in the 

 youngest embryo (fig. 3, ii.l.), has developed in a somewhat older specimen (fig. 4, jpn.) 

 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 forcturned, 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.tr.) ; afterwards they each send backwards a recurrent rod *. 

 The general appearance of the trabeculse, 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 Prcoral 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 LXXxr. 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 trabeculae 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, p.pg.) 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 subocuJar; 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 perichondrium ; and the most gelatinous part outside is the 

 rudimentary condition of the loose stroma or areolar connective tissue. 



