198 ME. W. K. PAEKEE ON THE STEUCTUEE AND DEVELOPINIENT 



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-growth by sending three prsenasal " suckers " (the cornua and prsenasal 

 rod) into the intermaxillaiy 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 axis things that are axial, and to the face 

 things that are facial. 



The ojcis appears to me to pass insensibly into the face in the internasal region, 

 although some may argue that even the trabecular cornua and praenasal rod are then 

 productions of the fore end of the axial elements. The somewhat lyriforra trabeculse 

 are of great breadth in the Selachians (compare PI. XXXV. figs. 3, 5, 6, t)\ with those 

 of the Salmon, /. c. pis. 1-4). 



In front they are shaped like pruning-hooks ; the blunt hook lookLag 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 j^!/), is equal to their 

 width. Behind, also, they do not meet, but apply their inner edge to the three foremost 

 notochordal " beads " (PI. XXXIX. 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 Menohranchus). The pterygo-quadrate 

 ends in front immediately below the fore end of the trabeculse ; in front of the trabeculae, 

 between the granular nasal sacs, the internasal tract grows broad behind and pointed in 

 front, the pointed tract being the rudiment of the azygous prsenasal cartilage. The 

 trabecular cornua are not at present solidified sufiiciently to show their distinctness 

 from the contiguous parts of the nasal capsules. 



Second Stage. Embryos of Bog-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 [yy) 

 lies behind the fore brain (C^, a), and the " middle trabecula " (m.tr) is not absorbed. 

 The true trabecula (tr) 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 



