784 
ME. W. K. PAEKEE ON THE STEUCTUEE AND 
by the lower tympanic lip of the basitemporal ( b.t .), which further inwards underlies 
the cochlear cavity ( c.l .), and at the mid line makes a secondary floor to the large open 
chink in the cranial floor — the “ posterior basicranial fontanelle ” (p.b.f). The cochlear 
otoconial deposit (Lot.) is seen, folded upon itself, in the end of the cochlear sac. 
A section made a little further backwards (Plate LXXXIV. fig. 4) shows the sudden 
lessening of the thickness of the investing mass (i.v.) behind the cochlear elevation (c.l.). 
The last section (fig. 3) was made in front of the notochord ; but here we come across 
its fore end (n.o.), which is enclosed in its own ectosteal sheath ; this bony deposit is seen 
to grow upwards and outwards into the surrounding cartilage (i.v.), and is especially 
developing itself below, where it forms the lower plate of the basioccipital (b.o.). We 
are here close behind the basitemporal, in the broadest part of the basioccipital (see 
Plate LXXXIII. fig. 1, b.o. , b.t.). This section and the next to be described, well 
show the triple nature of the basioccipital, and also teach how small a portion of the 
morphology of the skeleton is involved in the mere consideration of the form and 
number of the separate bones. 
The next section (Plate LXXXIV. fig. 5) is made where the skull-floor is thinnest — 
namely, just a little in front of the occipital condyle (see Plate LXXXIII. fig. 1, b.o., o.c.). 
External to the basioccipital bony centre (b.o.) with the enclosed notochord (n.c.), the 
cartilaginous investing mass (i.v.) first becomes thicker and then thins out again ; here 
it becomes enclosed within an upper and a lower ectosteal plate — the exoccipital ( e.o .) : 
this is more clearly shown in a more enlarged figure of this part of the section, given in 
Plate LXXXII. fig. 12. 
The rest of the section passes through the otic region (Plate LXXXIV. fig. 5), and 
shows the inner wall, imperfect below (pr.c.), the vestibular or upper otoconial mass 
(u.ot.), the ampulla of the anterior semicircular canal (a.sc.), the ampulla and arch of the 
horizontal canal (h.sc.), part of the tympanic cavity (ty.), and part of the squamosal (sq.). 
This bone is separated from the periotic mass by a quantity of the same kind of gela- 
tinous tissue as that which fills the tympanic cavity. The space between the squamosal 
and the periotic mass is very large, because of the deep angular fossa which runs along 
the outside of the anterior canal : this fossa is shown in Plate LXXXII. fig. 9, and in 
Plate LXXXIII. fig. 2, as well as in this section ; it may be called the “ upper periotic 
fossa.” 
Fifth Stage. — Head of Embryo 15 lines long ; 2nd Day after Hatching. 
The skull of the ripe chick (Plate LXXXIV. figs. 6-12) is interesting in many respects : 
it is profitable for comparison with the skull of the Fish, the Eeptile, and the immature 
Mammal ; and, looked at in its general form, it comes much closer to that of the nearest 
congeners of the typical Fowl than the skull of the adult bird. 
The types suggested by a view of the chick’s skull are the Llemipodiine, Pterocline, and 
Columbine outliers of the Gallinaceous Family (see Trans. Zool. Soc. vol. v. plates 
34-38): the skull of the adult Fowl has receded greatly from that of its lower and 
