194 MR. W. K. PARKER ON THE STRUCTURE AND DEVELOPMENT 
embryos nearly an inch in length, the skeletal tissues were acquiring a greater density, 
and the various folds of the skin were much more perfect. Yet over the third vesicle (C*) 
the integument was quite transparent, and the contents of the head visible through it. A 
lateral view (fig. 2), seen by reflected light in a specimen hardened both by alcohol and 
chromic acid, shows how the skin is acquiring its proper characters in its growth from 
below upwards. ‘The sense-capsule folds are closing; and the posterior edge of each 
postoral visceral arch has developed an opercular “vallance,” the fringe from the 
second being the counterpart of that which is so largely extended backwards over the 
hinder arches (branchials) of the Osseous Fish. Here the free labiate edge grows 
from the mandible and gill-arches, as well as from the hyoid arch. Arrested at this 
stage, they would leave the gills much exposed; but they close in to a great degree, 
” 
leaving only the well-known branchial slits. Morphologically considered, they also are 
the rudiments of such a growth of the skin as in an early stage covers over the visceral 
clefts in the ‘‘ Abranchiata.” No cartilage is developed at present in the substance of 
the mandibular arch at this part, merely a strong ligament which attaches the mandible 
to the skull between the trigeminal nerve and the apex of the hyoid arch. This liga- 
ment is the primary apex; the pterygo-quadrate is the secondary fork. A deep fissure 
is seen between the inturned end of the pterygo-quadrate bar and the olfactory sac (na). 
During growth the arcuate cleft between the first and second postorals has become a 
large triangular space, with the base above and the apex below. From the posterior 
edge of the upper part of the mandibular bar four clubbed filaments proceed ; they 
look upwards and outwards: these are the free eternal transitory gills of the mandi- 
bular arch, the precursors of the pseudo-branchia. The counterparts of these, growing 
out of the succeeding arches, all but the last, are four or five times as long as in the 
more immature specimen. ‘They are about ten on each bar, both on the right and left 
side. ‘The lower and upper views of this specimen (figs. 8 & 4) are very instructive ; 
and if the actual form of the enclosed bars of cartilage be held in mind (see fig. 1), it 
will be easy to understand their structure. The first cerebral vesicle (C*) is completely 
beneath the second (C2); and beneath the first the curious nasal sacs are seen, with their 
sigmoid valvular opening. The trabecular plate shows its form even in the opaque 
object. In front of the mouth are seen three lobes: the paired lobes contain the soft 
bulbous ends of the trabecular bars; and the azygous elevation between the nasal sacs 
contains the pranasal or basitrabecular cartilage, an unpaired commissural bar uniting 
the distal ends of the trabecular cornua. The solid side walls of the mouth contain not 
only the pedate pterygo-quadrate bar, but also the fourth upper labial cartilage ; this 
will be shown better in a more advanced stage. 
The lower jaw, seen from beneath, is a quadrilobate mass fixed behind and below by 
a broad short pedicle. Its external lobes are the angular and articular regions; and the 
submesial swellings contain the short Meckelian regions of this peculiar mandibular 
arch. The following arches are much more bowed out; and between them the visceral 
