200 MR. W. K. PARKER ON THE STRUCTURE AND DEVELOPMENT 
The most advanced specimens at this stage (1 inch 4 lines long) show much that is 
instructive, and fairly bridge over the space between the first and third stages. 
The head is fast becoming straight—see the nearness of the fore margin of the nasal 
sacs to the front of the head (Pl. XX XV. figs. 6, 7, as compared with fig. 5); and the 
trabecul, investing plates, notochordal sheath, auditory capsules, and visceral arches 
are all now chondrified. 
Ina horizontal view of a preparation in which the nasal and optic capsules were cut 
through, and the brain all removed, except in front (Pl. XXXV. fig. 6), we get a good 
view of the foundations of the Selachian chondrocranium. 
The notochord, which with the investing mass that has been cut through at some dis- 
tance behind the auditory capsules, is enclosed in a strong sheath of hyaline cartilage, 
has lost its beaded character in front, and now has pressed its end flat against the back 
of the pituitary body. 
The halves of the investing mass are scooped along their inner edges, where they 
cling to the sides of the notochord. ach plate passes some distance below the auditory 
capsule, but much more at both ends than in the middle. These cartilaginous bands 
have coalesced with the trabecule in front, growing into the lower edge of the thick 
transverse postpituitary wall (p.c/, py, iv, tr). 
The auditory capsule inside the anterior ampullar enlargement has coalesced with the 
thick outer end of the posterior clinoid wall. In front of the wall the trabecule dip, 
and are somewhat concave; behind, directly in front of the ear-capsules, each trabecula 
is growing upwards into an alisphenoidal crest, which runs forwards to the optic 
nerve (2). This is their narrow part ; further forward they expand ee the nasal sacs 
in a pedate manner, but do not yet meet at the mid line. 
The intertrabecular space is larger than its enclosing cartilages, and is only occupied 
at its end by the neck of the pituitary body. 
At the mid line, between the trabecule and the olfactory sacs, the granular semi- 
cartilaginous internasal tract is seen. I cannot discover that the tract is ever divided 
into two distinct bands of cartilage, although its counterpart always is so divided, below, 
in the Amphibia; in front it widens, curves right and-left round each nasal sac for some 
distance, and in the middle sends forward an azygous rod. 
This latter is the prenasal rostrum, the axis of the “ cutwater;” and the lateral 
growths are the cornua trabecule. Each of these is bilobate ; and in the next stage we 
shall see what a curious modification these two projecting masses of cells have 
undergone. 
At present it should be noted that the olfactory sacs, whose dome-like roof is now 
fast changing into cartilage, are very close together, only leaving a narrow valley 
between them, and leaving scant room for the septum or internasal region of the 
trabecule. 
When the head is examined from below we see the free forward growth of the 
