OF THE SKULL IN SH.IEKS 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 

 branchiae, 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 pUcm are arranged like the cogs 

 of 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 oesophagus is temporary, but shows what is possible in a low 

 vertebrate form ; long before the embryo escapes from the horny tendrilled pilloiv-case 

 the branchial slits are reduced to much smaller dimensions, relatively. The cog-shaped 

 plicse 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 

 {av) are seen to be about the size of the eye-balls (e), and to be ovoidal in shape ; they 

 are beginning to acquu-e 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 

 (6'^) 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 crazier is filled with delicate gelatinous tissue; 

 it is the transitory " middle trabecula " of Eathke. The notochord (ncj 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 subcartilaginous plate, the two 

 halves of which form the " investing mass " (iv) ; beneath the investing mass lies the 

 pharyngeal portion of the first branchial arch (br.l). 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 "), correspondmg to our tympano-Eustachian passage, is seen to be hiffh 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. 



