DEVELOPMENT OF THE SKULL IN THE PIG. 323 
above (@.s¢., h.s¢., p.sc.) and the cochlea below (cl.). ‘The hollowed tegmen tympani (¢.ty.) 
has in its hinder recess the head of the incus (7.); the recess ends in a round cup-like 
facet for the short crus of the incus, with its down-turned rounded head ;_ the “ aceta- 
bulum” for this head is finished, externally, by the squamosal, part of which, having 
become adherent, is shown in the figure. Below this wall-chamber for the incus is the 
fenestra ovalis with the enclosed stapes (f.ov., st.); the long axis of the oval space and 
oval base of the stapes is upwards and forwards. ‘The inturned hook of the long crus of 
the incus is now coupled to the neat head of the stapes by means of an intermediate 
bone, the “ os orbiculare ” (0.0b.), a special centre developed in the primary head of the 
second postoral bar, which, limpet-like, applied itself to that periotic “bud” which 
became the stapes, by a process similar to that which detaches the axillary buds in Leliwm 
tigrinum. In this figure the malleus is not given; it is shown in fig. 5 (ml.). The 
processus gracilis ( p.gr.) is reduced to a style, ending in fibrous tissue; the manubrium 
(mb.) is flat and slightly arcuate; the “head,” articulated with the incus, is elegantly 
notched for this purpose, and fits on to the incus by a synovial joint, the miniature of 
that by which the tibia fits on to the astragalus in this same animal. Between the 
head and the manubrium the bone is thin, and is scooped externally ; the head sends 
inward a rounded process (¢.p.7.), and the manubrium sends backwards an angular snag ; 
this latter is for the attachment of the “ tensor tympani” muscle. ‘The little secondary 
nucleus of cartilage which we saw developed between the dislocated incus and stylohyal 
(Plate XXX. figs. 8 & 9, d.hy.) is now attached to the neck of the stapes by its broad 
outer end, whilst its bluntly pointed distal end is buried in the fibres of the tendon of the 
“ stapedius’”’ muscle (st.m.). ‘This is the /ast effect of the high degree of metamorphosis 
exhibited by the second postoral bar of the Mammal. ‘The fore edge of the exoccipital, 
with its paroccipital spur (fig. 2, ¢.0., p.oc.), is strongly clamped upon the auditory capsule ; 
this is also made still stronger by the large posterior flange of the overgrowing squamosal, 
? 
not shown in this figure. 
The under surface of the snout is also given at this stage, to show the complete 
coalescence of the ale nasi with the recurved trabecular horns, and their continuation 
backwards as the ‘recurrent lamine” (rc.c.), also the alinasal external segment or 
“appendix.” ‘The solid fore end of the snout, already full of small blood-vessels, is 
ready to become the snout-bone for rooting ; this bone is formed in the ossified knees 
of the trabeculee: the stunted, recurved prenasal cartilage is now undistinguishable 
from the base of the septum nasi, formed by the complete coalescence of a large tract of 
the trabecular bars, the long commissure of the foremost facial arch. 
Eighth Stage-—The Skull of a Pig 6 months old. 
This makes a more convenient /ws?f stage than the adult, as here are still in existence 
the greater number of the sutural landmarks, so soon to be largely obliterated, whilst 
the change in general form is rather of interest to the zoologist than to the morpho- 
logist. The long angular skull (Plate XXXVI. fig. 4, and Plate XX XVII. figs, 1 & 2) 
2U2 
