OF THE SKULL IN SHARKS AND SKATES. 195 
the skin was of extreme thinness, and composed of very delicate cells, over the greater 
part of the /ong 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 
examined 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 papill, as in the unhatched Tadpole (“ Frog’s 
Skull,” pl. 3, figs. 2 & 3, br 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 /ateral 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 different from what 
is seen in the larval Frog at a similar stage (“* Frog’s Skull,” pl. 3. figs. 3, 2, mn, & pl. 4. 
figs. 1, 2, mn), where the first postoral clings close to the trabecula. Afterwards, in the 
third stage, these bars, in the Frog, diverge; and then (fig. 7, 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, #7); 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,” pl. 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 isa 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 adyanced specimens (Pl. XXXIV. figs. 2-5), delicate thread-like 
