198 MR. W. K. PARKER ON THE STRUCTURE AND DEVELOPMENT 
On afterthought, it is an anomaly to me that the foremost pair of visceral or “ pleural ” 
rods should grow straight forwards as the head straightens, and then from their upper 
edge develop three fourths of the chondro-cranium—namely the posterior and anterior 
sphenoidal regions, the ethmoidal or proper olfactory region, the internasal region in 
front of the tract supplied by the olfactory nerves,—and then finish off this exuberant 
and varied skull-erowth by sending three preenasal “ suckers” (the cornua and prenasal 
rod) into the intermaxillary region. 
There must be here some suppression of originally distinct parts, or elements ; and it 
seems to me now to be safer to give to the avis things that are axial, and to the face 
things that are facial. 
The ais appears to me to pass insensibly into the face in the internasal region, 
although some may argue that even the trabecular cornua and prenasal rod are then 
productions of the fore end of the axial elements. The somewhat lyriform trabeculé 
are of great breadth in the Selachians (compare Pl. XXXYV. figs. 3, 5, 6, tr, with those 
of the Salmon, /. ¢. pls. 1-4). 
In front they are shaped like pruning-hooks; the blunt hook looking towards its 
fellow behind the internasal tract, but not meeting it. The back of the blade looks 
forwards and outwards, lying close behind the olfactory sacs. Their interspace, which is 
largely occupied by the infundibulum and pituitary body (inf, py), is equal to their 
width. Behind, also, they do not meet, but apply their inner edge to the three foremost 
notochordal ‘“ beads” (Pl. XX XIX. fig. 6). They send a right-angled wedge between 
the front of the investing mass and inner face of the ear-sac, to which they cling, and in 
front of which they form a rounded elbow: hence the outer edge of each plate is 
deeply notched in a semioval manner. Externally, the front projection is the rudiment 
of the lateral ethmoid, the part to which the antorbital or ethmo-palatine cartilage is 
attached in some Selachians, in Teleostei, in Urodeles, and in Anura. 
The hinder elbow is the part to which the “ pedicle of the suspensorium ” is attached 
in the Amphibia (“ Frog’s Skull,” and Huxley on Wenobranchus). 'The pterygo-quadrate 
ends in front immediately below the fore end of the trabeculz ; in front of the trabecule, 
between the granular nasal sacs, the internasal tract grows broad behind and pointed in 
front, the pointed tract being the rudiment of the azygous prenasal cartilage. ‘The 
trabecular cornua are not at present solidified sufficiently to show their distinctness 
from the contiguous parts of the nasal capsules. 
Second Stage. Embryos of Dog-fish 14-16 lines in length. 
At this stage the embryo of Scyllium canicula still retains the “ mesocephalic flexure,’ 
but the brain (PI. XXXVI. fig. 2) has become very complex. ‘The pituitary body (py) 
lies behind the fore brain (C1, a), and the “ middle trabecula” (m.t7) is not absorbed. 
The true trabecula (#7) is very much enlarged, and flattening out above and behind has 
begun to form the large flat floor on which the fore part of the brain-sac rests; the 
