752 PROFESSOR W. C. M'INTOSH 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 copulse for the 

 respective arches. 



In T. 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 a symphysis is formed at the anterior margin 

 (PI. 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 fantastic 

 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 struc- 

 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 (PL XIX. fig. 10), and even in a newly-hatched fish (ss, PL VIII. fig. 6), 

 above the mid-brain, mb (optic vesicles), it is small, though larger in front (between the 

 nasal capsules), fb, and behind, hb (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 

 a very 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. beiDg probably such an example ; 

 but under ordinary conditions the enlargement is considerable, and median sensory papillae 

 appear in it, immediately beneath the corneous layer, with connecting nervous filaments 

 passing downward, probably to the lateral sensory tract (sno, PL XVII. fig. l). 



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 



