OF THE SKULL IX SHAEKS AND SKATES. 193 



the skin was of extreme thinness, and composed of very delicate cells, over the greater 

 part of the long posterior vesicle (C^). 



Each sense-capsule was seen, externally, to be formed by an infolding of the skin of 

 the embryo, the " epiblast " and adjacent layer of " mesoblast." 



Behind the great gaping oral opening there were six clefts, equidistant from 

 each other, and not meeting below. This embryo, being treated with carmine, and 

 exammed in glycerine as a transparent object, showed well the cartilaginous pith 

 inside the thick ridges that intervene between the clefts. The surface of these ridges 

 had budded into a series of rounded papillae, as in the unhatched Tadpole (" Frog's 

 Skull," pi. 3, figs. 2 & 3, Jr 1 & 2) ; these were the beginnings of the external gills ; they 

 were found on all the " postoral " arches except the last. In the Frog-embryo they 

 were not present on the first and second postorals. The object (fig. 1) has been drawn 

 obliquely, exactly as it appeared to the eye under the microscope ; the other figure (2) 

 shows the exact lateral view of the parts. All but the first two postoral ridges turn 

 directly inwards (fig. 6) ; the enclosed pith is a stout sigmoid rod of young hyaline carti- 

 lage. Such a pith exists in the first two postorals {mn, hy); but they send forwards 

 from their point of attachment a large pedate process ; they are subbifurcate above. 

 This anterior fork, in the case of the second postoral, or hyoid arch, applies itself along 

 the side of the auditory capsule. The first postoral, or rudimentary mandible, is still 

 more produced beyond its proper suspensorial point ; its foot or fork grows forwards 

 over the mouth, and meets its fellow of the opposite side below the eye and behind the 

 nasal sacs. Here we have the first rudiment of the " quadrato-pterygoid " arcade ; it is 

 found in the " maxillo-palatine " rudiment of the embryo. This is difi'erent from what 

 is seen in the larval Frog at a similar stage (" Frog's Skull," pi. 3. figs. 3, 2, mn, & pi. 4. 

 figs. 1, 2, inn), where the first postoral clings close to the trahecula. Afterwards, in the 

 third stage, these bars, in the Frog, diverge ; and then (fig. l,p.pg) they become united 

 by a conjugational band, the first rudiment of the large palato-pterygoid bar. Lying 

 deeper within the tissues, in reality in the oral roof, we see the edge of a bar in the 

 embryo Shark (PL XXXIV. figs. 1 & 6, tr) ; it is the trabecula. 



The seven pairs of free visceral rods undergo a large amount of metamorphosis by 

 segmentation, so as to form a most flexible oral and branchial apparatus. Already, 

 over the infoldings of the young eye-ball, a ridge is seen ; this is an important part of 

 the skull when developed, namely the " superorbital band " (see " Salmon's Skull," pi. 4. 

 figs. 1-3, s.ob). The ear-sac is still on the same morphological level as the nasal sac and 

 the outworks of the eye-ball ; it is a rounded fold, which soon will nearly close, however, 

 and chondrify beneath the skin. The very rudimentary nasal sacs cleave close to the 

 inferior surface of the depending fore brain. The mouth, notwithstanding the palatal 

 foot-like process that passes over it, is very open and gaping ; altogether the postoral 

 bars and clefts make an " open-work " of the whole of the mouth and throat. 



In the more advanced specimens (PL XXXIV. figs. 2-5), delicate thread-like 



