DEVELOPMENT OF THE SKULL IN THE SALMON. 
137 
of the trabecular horns, and the width and thickness of the subnasal lamina just in front 
of the facet for the palatine ; the skull now at this part is very Acipenserine. In this 
section the nasal sacs are severed at their opening, the lateral parts of the premaxillaries 
are cut through, the fore part of the parasphenoid, and the “ prepalatine ” bar with its 
ectosteal plate. Another section (Plate V. fig. 4), made near the fore part of the orbits and 
a little obliquely, shows on one side part of the antorbital (ectoethmoidal) plate, and on 
the other the roof-cartilage. This is a most instructive section, and should be compared 
with a similar section of the Fowl’s skull made at the beginning of the third week of 
incubation (“Fowl’s Skull,” Plate lxxxiii. fig. 11 ,p.s., i.o.s., b.s.) : for the crest growing- 
down from the roof is the “ mesoethmoid” passing into the presphenoid; the roof itself 
ends in the Bird in a spike above the olfactory groove, and in the young Salmon in the 
free, retral, median lobe. Below, the tilted coalesced trabeculse send up their crest, in 
which the lower part of the “ mesoethmoid ” passes below the presphenoid into the basi- 
sphenoidal region ; this will be better understood in the next stage. Here we see the 
raised middle of the “ parasphenoid ” applying itself to the trabecular groove and the 
tilted and broad “ mesopterygoid” region of the subocular arch (pa.s.,pa.). In another 
section (Plate V. fig. 1), made through the fore part of the auditory capsule, we see the 
ampulla and part of the arch of the anterior canal ( a.s.c .) overlying the hyo-mandibular 
(the fore part of which is cut through), and forming the tegmen tympani. ITere the skull 
is widest, for it covers in the posterior part of the middle lobe of the brain, where it 
overlaps the posterior region ; here was seen, inside the ampulla of the anterior canal, 
the Gasserian ganglion (5). The posterior branch of this fifth nerve is seen passing- 
through its own foramen in the “prootic” cartilage; and the fore part of the investing- 
mass, confluent with the ear-cartilage, is undergirded by the “basitemporal wings” of 
the parasphenoid, over which lie the orbital muscles ( o.m .). Here also is well seen the 
manner in which the periotic cartilage is folded over the semicircular canal, but fails to 
wall it in ; the sharp edge above is that which runs into the boundary-band that comes 
from the sides of the cartilaginous roof. 
Seventh Stage. — Young Salmon of the first summer , 1 \ to 2J inches in length. 
We have now come to a stage in which the Ganoid characters are being rapidly effaced, 
whilst that which is “Teleostean” has become apparent. In this arbitrary but not non- 
natural seventh age of the Salmon we are able to detect not only the intermediate laminae 
of bone in which the cartilaginous skull is arrayed, and by which, as in proven armour, 
its possessor takes a high ichthyic rank, but we have now rudiments, at least, of all the 
deep laminae that graft themselves upon the cartilage within. The cartilaginous skull 
itself has become much more massive, and it is no hard task now to harmonize the skull 
of the young with that of the adult. Holding in mind the condition of the last stage 
(Plate V. figs. 1, 2, 3, 4), we see that the cartilaginous roof is increasing in thickness, and 
extending backward over the middle cerebral region, whilst the sharp edges of cartilage 
(behind from the occipital ring, and postero-laterally from the cartilaginous ear-sacs) 
