DEVELOPMENT AND LIFE-HISTORIES OF TELEOSTEAN FISHES. 749 
actual slits, a phenomenon not seen until long after the arches are fully differentiated. 
From this protrusion of enteric hypoblast, Sepcwick likened these paired pouches to 
nephridia ; indeed, he considers them homologous, the kidney-system of vertebrates never 
overlapping them, but commencing behind their posterior limits (No. 148, p. 67). The 
clefts or gill-openings are probably not formed until some time after extrusion from the 
egg, but the hypoblastic diverticula indicate their future position, and the dense meso- 
blastic masses between them form the branchial skeleton or gill-arches. The latter in 
their early condition appear as a series of rounded or subquadrate structures, when seen 
from below (PI. VIII. fig. 5), but viewed from the side fine striations merely are observed 
passing dorso-ventrally with a slight inclination forward, these striations being the linear 
outpushings of the oral hypoblast (P]. VIII. fig. 6). The arches thus early indicated are 
not simultaneous, and LEREBOULLET observed in the embryo of Perca that they appeared 
successively from behind forward (No. 93, p. 616). The precise stage when the branchial 
clefts are open cannot be stated. There is no doubt it is very late, for long after the 
arches are clearly defined the slits are still unformed, even in so advanced an embryo as 
Gastrosteus (Pl. XI. fig. 9), in which the mouth is open, while the hypoblastic cells, hy, 
which pass down between and surround the bars, bra, still form a continuous layer. A 
fifth branchial arch can be made out, but remains rudimentary in the Gadoids and other 
forms here considered ; while anterior to the four branchial arches proper, two pairs of 
stout bars are developed at an early stage, viz., the hyoid (hy, Pl. X. figs. 2, 3; 
Pl. XIII. figs. 5, 6), and in front the mandibular (mn,—MeckeEv’s cartilage). Both these 
arches undergo a more complex development than the branchial rods behind, and with 
the appearance of cartilage-cells, both are readily distinguished by their greater length 
and stoutness, as well as by their direction, both extending forward and tending rapidly 
to complete the arch on the floor of the mouth. The upper portion of the first or man- 
dibular arch becomes expanded (PI. IX. fig. 6; Pl. XIII. fig. 5); and Parker speaks of 
it as well marked in the salmon (No. 117, p. 113), splitting longitudinally into two, 
giving origin in this way to a fore part, the mandible proper, and a hind portion, the 
hyoid. In our forms the hyoid is already well developed when the division of the man- 
dibular cartilage takes place, and it would appear therefore that the posterior portion, hm, 
which is the stronger, and much expanded at its upper extremity, is really the hyo- 
mandibular, thus arising as an element separate from the hyoid, while the narrower 
anterior part, pg, also split off, becomes the palato-quadrate. Before this splitting is 
complete, the extended lower part separates as the primary lower jaw or mandible, mn, 
and its proximal part becomes enlarged (PI. IX. figs. 6,7; Pl. X. figs. 2, 3), to afford 
an articulating surface for the two suspensory elements above, the palato-quadrate and the 
hyomandibular, which separately articulate, the former doing so earlier than the latter, and 
more directly (Pl. IX. fig. 7). From the proximal portion of the mandible an anterior 
process grows out at a subsequent stage (Pl. IX. fig. 7), while in the angle below the 
end of the stout and broad hyomandibular, a small element, the angular, develops. The 
forward growth of the palato-quadrate cartilage must be a late phenomenon, for the pre- 
