752 PROFESSOR W. C. M‘SINTOSH AND MR E. E. PRINCE ON 
In the young cod, three weeks after hatching, the branchial system is wholly converted 
into cartilage, and forms a complex series of translucent hyaline bars, in which the four 
parts—the epi-, cerato-, hypo-, and basi-branchial pieces can be distinguished, and the 
small rod-like azygos pieces in the middle of the oral floor form the several copulee for the 
respective arches. 
In 7. gurnardus, and other pelagic forms, the cartilages of the jaws apparently 
become stiff and immobile about the eighth day after hatching, and the hyoidean 
apparatus also shows no regular movements. The fish, however, by its forward jerking 
motion, drives water into the widely-open mouth, and aeration is thus easily effected, for 
the opercular opening is broad, and the operculum itself projects outward and backward, 
as a thick flap of the integument. 
The mandibular rami, mn, continue to lengthen upon each side to such a degree that 
they project much beyond the upper jaw, and asymphysis is formed at the anterior margin 
(Pl. X. figs. 1-5). No feature is more striking than this extraordinary development 
of the lower jaw, and in sickly and abnormal embryos it produces the most fantastie 
appearances—the protruding mandibles frequently curving downward, so that the gape 
of the young fish is remarkably wide (Pl. XIV. fig. 2), and even in normal examples this 
extension of the floor of the mouth, and the mobility of the lingual and hyoidean struec- 
tures, increase the oral aperture very much (Pl. X. figs. 1-3, 5, 5a), and contribute 
doubtless to facilitate the capture of the minute organisms which form the earliest food 
of the young Teleostean. 
Skull.—The capsule enclosing the brain is, like the rest of the body of the embryo, 
simply a thin epiblastic layer composed of the flattened corneous stratum, and the thicker 
sensory remnant beneath. 
Between this ectodermal covering, ep, which though expanded in the form of 
a bulbous protective capsule, can scarcely be regarded as a cranium, and the brain-mass 
below, a space intervenes occupied by a transparent substance, apparently of a jelly-like 
consistency. This space, ss, filled with fluid, is inconsiderable during the earliest stages 
within the ovum (PI. XIX. fig. 10), and even in a newly-hatched fish (ss, Pl. VIL. fig. 6), 
above the mid-brain, mb (optic vesicles), it is small, though larger in front (between the 
nasal capsules), fb, and behind, Ab (over the cerebellum and fourth ventricle); but it in- 
creases at the end of the first week (Pl. VIII. fig. 7), and during the second or third week 
after extrusion it becomes enormously enlarged, and imparts to the more advanced embryos 
avery grotesque appearance (ss, Pl. XII. figs. 2,6; Pl. XVI. figs. 1, 3,5). Often this 
sub-epidermal enlargement abnormally develops, and embryos with the cephalic region 
remarkably swollen are not uncommon—fig. 3, Pl. XVI. being probably such an example ; 
but under ordinary conditions the enlargement is considerable, and median sensory papillze 
appear in it, immediately beneath the corneous layer, with connecting nervous filaments 
passing downward, probably to the lateral sensory tract (sno, Pl. XVII. fig. 1). 
At an early stage the mesoblast of the head consists of a thin stratum chiefly aggre- 
gated between the eyes and the neurochord (mes, Pl. IV. fig. 17), while, above, the brain 
