No. 3.] MUSCLES AND NERVES IN AMIA CALVA. 515 
of the future process, through which there is a large open chan- 
nel which, owing to the shape of the base of the brain and basis 
cranii, runs decidedly upward as well as forward, almost at right 
angles to the olfactory nerve. Following this stage the outer 
edge of the open channel near its posterior end grows upward, 
then inward, and then inward and downward toward the inner 
edge of the channel with which, when the larvae is about 10 
mm. in length, it has coalesced, forming a narrow arched bridge 
over the posterior end of the channel. Under the bridge thus 
formed from its inner wall, the former inner edge of the open 
channel, the oblique muscles have their origin, and through the 
large opening or canal, formed by the bridge, a large venous 
vessel passes from the nasal to the orbital regions of the skull. 
Across the anterior, open end of the canal the olfactorius runs, 
nearly at right angles to it. The canal and the open channel 
in front of it therefore represent the extreme, upper, anterior 
end of the orbit, the orbito-nasal fenestra, and that part of the 
olfactory canal that in the adult lies in front of that fenestra. 
In later stages the preorbital process thickens, grows upward 
along the side of the brain and then backward, forming the 
beginning of the roof of the orbit. This is the condition in 
larvae 12 mm. and 14 mm.in length. The lobus olfactorius at 
these stages lies practically in front of the preorbital process, 
for the transverse vertical sections that, in 12 mm. specimens, 
pass through the process, pass also through or behind the reces- 
sus interolfactorius. The side wall of the skull in front of the 
process has at these ages not yet been formed, but, immediately 
in front of the lobi, a vertical transverse ridge, or a median pier, 
marks the anterior end of the cranial cavity. The olfactory 
nerve, on each side, arises from the lower, outer, anterior sur- 
face of the lobus, and runs downward, outward, and slightly 
forward along the shelving lateral edge of the basis cranii. 
In larvae 20 mm. in length the preorbital process on each 
side has grown forward, enclosing the orbito-nasal canal, and 
that canal now opens on the shelving olfactory region of the 
basis cranii under or immediately in front of the anterior end of 
the lobus olfactorius. Where it opens in front of the lobus a 
true olfactory foramen is formed or indicated, lying between the 
