DEVELOPMENT OF THE SKULL IN THE SALMON. 
139 
way — namely, by the extension backwards below the optic nerves of the “ trabecular crest.” 
In the Bird this crest (“Fowl’s Skull,” Plate lxxxiii. fig. 14, b.s.) expands in front of 
the pituitary body into the “ anterior clinoid wall” (Plate lxxxiii. fig. 2, a. cl.). In the 
Fish the trabecular crest runs into the pituitary space, between the out-bowed part of 
the trabeculae (Plate V. figs. 6 & 7). Thus the foundation of the clinoid wall is laid in 
cartilage, but the wall itself is finished in another way. The membranous septum behind 
the optic nerves becomes partially ossified ; this osselet is a Y-shaped little prop, the arms 
of which seize the lower edge of the alisphenoids, and the leg of which is implanted 
upon the rounded end of the trabecular crest (Plate V. figs. 6 & 7, Plate VII. figs. 3 & 4, 
and Plate VIII. figs. 2, 3, 4, b.s.). At first (Plate V. figs. G & 7) the foot of the bone 
does not reach the cartilage, but they grow towards each other afterwards. This bone 
is a prepituitary “ basisphenoid it is a deep lamina or “ ectostosis,” and it becomes one 
with the primordial skull both above and below. Immediately behind this bone the 
pituitary body (Plate V. figs. G & 7 -,py-) descends to reach thej 4 parasphenoid ’ there 
is no other seat to the “ sella turcica;” and on each side the internal carotid artery (i.c.) 
enters. 
Before leaving the fore part of the skull I may refer to the extraordinary expansion 
which the trabeculae have already undergone. At first (Plate I. figs. 1 & 2, tr.) they were 
filiform thickenings ; they soon (fig. 5, tr.) spread into a bifoliate form ; the “ bifoil ” has 
already become differentiated into a “ meso-” and an “ cctoethmoidal ” region. Now see 
what has taken place ! Between the eyes we have the grooved interorbital base 
(Plate V. fig. 9), then the wide floor of the “ ectoethmoid ” with the “ palatine facets” 
then the floor of the nasal sacs, and, lastly, the emarginate trabecular plate formed by 
the trabecular horns (ethmo-vomerine cartilage of Huxley). Above, the lateral parts 
have grown into the sloping prefrontal wings, which meet in the roof-cartilage , whilst 
along the mid line there grow backwards those two most important crests, the “ ethmo- 
presphenoidal ” and the “ trabecular.” 
We never see a cranium pure and simple ; for the outgoings and incomings of 
nervous force there are required appropriate organs that either nestle under the eaves 
of the skull or are projected into its walls. All the space from the great fifth nerve to 
the compound eighth is occupied by the antechambers, chambers, and galleries of the 
ear-organ ; here no fence but membrane and a gelatinous stroma is interposed between 
the brain and the labyrinth. But the skull on the outside becomes exceedingly strong 
by this impaction of the organs of hearing ; the rudiments of the thick bony blocks that 
exist in this part of the adult are now to be seen as their “ ectostoses.” But the audi- 
tory region is fringed on its anterior margin by a slight cartilaginous growth, the thick 
upper selvedge of which is formed by the “ superorbital band this fringe is under- 
going ossification, and is the “ alisphenoid as I have just mentioned, it is underpropped 
by the Y-shaped bone. This is a very unfinished skull as far as the sphenoids are con- 
cerned, and has little in common with what we are all familiar with in the human skull 
and in that of the Mammalia generally. In the Mammalian skull all that part of the 
