DEVELOPMENT OE THE SKULL IN THE MAMMALIA. 
33 
cartilage here drawn as taken off the auditory capsule shows the horizontal and most 
of the posterior canal ( h.s.c., p.s.c .) exposed in the tube of cartilage. Winding round 
the inside of the incus, where it has been cut off from its proximal part in the cranial 
wall, we see the facial nerve (VII.) forming an elegant arch nearly parallel with that 
of the horizontal canal. In front of its imbedded part in the figure there is an oblong 
tract of cartilage, this is the part where the epihyal ( e.hy .) has coalesced with the 
capsule, and from this it has been cut away in this slice. Below this the facial nerve 
is seen to send upwards a fork, suddenly ; this is considerably smaller than the stem of 
the nerve, and in the uninjured skull rides over the epihyal to get inside the manu¬ 
brium mallei; this is the “chorda tympani ” nerve (VIP.), forking over the visceral 
cleft to join the hindmost division of the 5th nerve. 
Dissection of the Skull of Tatusia peba. —Fourth Stage {embryo, 3 inches long). 
The dissected skull of Tatusia peba shows some differences from that of T. hybrida 
that are specific, yet most of the changes now to be described are due simply to 
advanced growth. 
Here the aim nasi, nostrils, and narial valves (Plate 5, fig. 5, al.n., e.n., n.v.) are like 
what I have just described, but the bones just behind these cartilages are larger than 
we should find them in a species of T. hybrida of the same age ; these are the pre- 
maxillaries (px.) and the foremost paired vomers ( v'.). Still they are all distinct and 
characteristic, the premaxillaries being small, oblique, notched, lateral shells of bone, 
with no palatine portion, only with their lower edge curled in. The front paired 
vomers (v'.) are twice as large as in the last, and are long shells of bone, with their 
oblique hollow face turned outwards, towards “Jacobson’s organ.” 
Interdigitating with these four bones we see the two maxillaries (mx.), large, 
inferiorly placed tracts of bone, showing the infraorbital foramina (V 3 -) on their under 
surface, halfway from their swollen shoulder to their suborbital notch. 
I find several alveoli, with their teeth, on the hinder two-thirds of their lower 
margin, and then a flange from the inner alveolar wall growing into the palatal region, 
as wide as the rest of the hard palate, but separated from it by a very distinct multi- 
perforate groove. The jugal process is wide, and is clamped by the angular fore part 
of the jugal (j.) which widens, inwards, at its middle part, and then becomes narrow 
where the zygomatic process of the squamosal rides upon it. 
A very long sutural line runs along the palate from the ake nasi {al.n.) to the 
presphenoid {p.s.) ; in front this is formed by the front paired vomers ; in the middle by 
the maxillaries ; and behind by the palatines (pa.). Their mutual suture is two-fifths 
the length of that formed by the palatine plates of the maxillaries ; the palato-max illary 
suture is arched forwards. The palatines do not narrow in so much behind, as in 
the last (Plate 2, fig. 6), and they also are grooved to, and beyond, their foramen. 
The thick half-coiled pterygoids (pg.) also do not run so far under the skull, and they 
MDCCCLXXXV. F 
