168 MR. W. K. PARKER ON THE STRUCTURE AND 
stems of the orbitosphenoids (0.s.) show their optic perforations (IT.), close behind 
the outer margins of the cribriform plate; thence the cartilage expands rapidly, 
and curves over the sides of the roof as far inwards for some distance as the 
lateral ethmoidal lobes (a/.e.); the line of union between these parts is still visible. 
Narrowing from before backwards, and bowing outwards, the cartilage runs so as to 
become, first, the supra-auditory (s.a.c.), and then the supraoccipital region (s.0.) 
The neat, rounded selvedge of this cranial wall and roof (tegmen cranii) forms the 
outline of a huge cruciform upper fontanelle, through which, the membrane being 
removed, we see the floor of the cranial cavity. Much of what has been described in 
the lower view is seen here from its upper face, but the low postclinoid wall (p.cl.) 
and the large multiperforate meatus internus (VIL, VIII.) are only to be seen on this 
face. Also the general smoothness of the gently concave inner surface of the 
chondrocranium is to be noted, and, over the top, the manner in which the supra- 
auditory part of the tegmen cranii flanks the fore edge of the occipital roof (s.0.), a 
sulcus marking the distinct regions. 
Fourth Stage (continued).— Visceral arches of the skull of an embryo Mole ; £ inch long. 
J D Y 5 g 
A. somewhat more advanced embryo than the last yielded me a very important 
stage in the development of the visceral arches. The deep and the superficial jaw 
are shown in relation, with the hinge-piece (incus) attached to the fore part of the 
auditory capsule. In this inner view (Plate 28, fig. 2) the ampullz of the anterior 
and horizontal canals (a.s.c., h.s.c.) are laid bare, and the short crus of the incus (?.) 
is seen to be attached close in front of these parts of the membranous labyrinth ; that 
process is very short and obliquely attached ; and in this shortness and obliquity 
it shows an intermediate stage between the normal Mammalian incus, on one hand, 
and the curiously arrested incus of a Monotreme, on the other. The long crus, 
however, is well developed, and is articulated by its inturned discoidal end, with the 
head of the stapes (s¢.), the base of which is turned towards the eye. The malleal 
end of the deep mandible is well developed, and the fore-turned internal angular 
process lies in the centre of a thick cushion of soft stroma—the future membrana 
tympani (i.ty.). This is partly enclosed by a delicate lunule of bone, the annulus 
tympanicus (a.ty.). The posterior angular process of the malleus is almost suppressed. 
The main part of MEcKEL’s rod (mk.) is evenly terete and sinuous, but it is largest 
near the head, and near the distal end; there it is continued into a median process, the 
basimandibular (b.mn.), into which both the rods end. The only bony matter in this 
primary mandible, as yet, is a short ring, or shaft, close behind the thick part, close to the 
median rod. Morphologically speaking, this is a hypobranchial bony segment. Outside 
all but the malleal portion there is, already, a well-formed superficial jaw, bony in front 
and cartilaginous behind, and having a groove, on its inside, between its condyloid 
and angular processes (ed.p., ag.p.) for the descending Meckelian rod, the bony 
matter (d.) beginning to run up the unciform coronoid process (c.p.). Here, if there 
