OF THE SKULL IN SHARKS AND SKATES. 195 
clefts are more extended, both above and below; their edges also develop a more 
distinct opercular flap. The huge pharyngeal bag is now seen to be slit on both sides; 
six of these slits are developed, and they are partly filled in by a very beautiful 
growth of plicated skin and of long clavate filaments. These filaments, the external 
branchie, are now seen to arise from the hinder edge of the bars, and to escape from 
the clefts like the contents of dehiscing carpels. The plice are arranged like the cogs 
ot a wheel; they occupy both sides of the first four branchial arches, the hinder side 
of the hyoid arch, and the fore edge of the last branchial. This open condition of the 
respiratory region of the cesophagus is temporary, but shows what is possible in a low 
vertebrate form ; long before the embryo escapes from the horny tendrilled pillow-case 
the branchial slits are reduced to much smaller dimensions, relatively. The cog-shaped 
plicee on these semicircular bars are the rudiments of the permanent or internal gills; 
they are hidden; and the bowed railings are filled in by the extension of the retral 
opercular folds. In the under view the umbilicus is shown, and on each side of it the 
rudiment of the pectoral fin; the heart is in the angular space between and below the 
posterior branchial arches, in front of the umbilicus. In the upper view, the ear-balls 
(au) are seen to be about the size of the eye-balls (¢), and to be ovoidal in shape; they 
are beginning to acquire their own cartilaginous covering. The brain-sac is at present 
almost entirely membranous; and both the skin (cuticles and cutis) and the stratum 
of cells beneath that splits into dura mater and cartilaginous skull (roof and wall) are, 
in the upper region, exceedingly thin and diaphanous. 
One of the most important views of the structure of this early stage of the Shark’s 
skull is obtained by making a solid vertical section to be viewed as an opaque object 
(see fig. 5). Now the thinness of the integument over the third brain-vesicle (C?) can 
be demonstrated ; and this vesicle is largely filled by a thin fluid to two thirds its 
depth ; the interior of the other vesicles is very soft and diffluent. The middle vesicle 
(C?) is very bulbous; and the anterior (C!) is now developing into the hemispheres. 
Above and behind the fore brain is the pituitary body (py); and it helps to enclose a 
space formed by the curvature of the neural axis at its cephalic end (mesocephalic 
flexure); this cavity in the hook of the crozier is filled with delicate gelatinous tissue ; 
it is the transitory “middle trabecula” of Rathke. The notochord (nc) follows the 
elegant curves of the neural axis where it passes into the hind brain; it reaches to the 
pituitary body. On each side of the notochord is a stibcartilaginous plate, the two 
halves of which form the “investing mass” (iv); beneath the investing mass lies the 
pharyngeal portion of the first branchial arch (47.1). This section well shows how the 
pharynx is railed in by the visceral bars, and that the mucous membrane is folded into a 
saw-like series, the teeth lying on the inner face of the bars above. The triangular open- 
ing (“spiracle ”), corresponding to our tympano-Eustachian passage, is seen to be high and 
short, unlike its successors. The way in which the mandibular stem has been, as it were, 
trained forwards, like an espalier, to the front of the mouth, is also clearly shown. 
