DEVELOPMENT AND LIFE-HISTORIES OF TELEOSTEAN FISHES. 747 
chordal sheath, and BaLrour refers to both layers as closely adherent, though distinct, but 
the former apparently decreases in thickness, and is then ditlicult to see (No. 15, vol. xi. 
p. 421). From the mesoblastic perichordal sheath alone the vertebral bodies originate, 
while its outer limiting stratum (the elastica externa) gives origin to the arches. The 
neural arches precede the hemal in development; uo trace, in fact, of the ventral 
processes being discernible when the neural arches project some distance dorsally. 
Of course, in a degenerate skeletogenous layer, such as the Teleostean perichordal 
sheath, the identification of the precise layers, seen more favourably in other fishes, is 
attended with much difficulty ; and one of us, in attempting to distinguish the different 
lamin, has referred to the outer layer as a “ limitans externa” (No. 122, p. 454) ; indeed, 
the opinion expressed that the existence of an “ elastica externa” in Teleosts, is a doubt- 
ful point, is supported by the fact that such a membrane does not properly exist in 
Amphibians, as well as in the Amniota. Favourable sections of Teleostean embryos, 
especially such a form as Cyclopterus, bear out, however, the above interpretation, the 
external layer being very distinct. Outside the perichordal sheath itself in post-embryonic 
stages plates of spicular substance develop. Thus in a young but mature specimen of 
Pleuronectes, the oral end of the notochord is seen to have acquired such a spicular 
sheath—formed apparently in the connective tissue outside the external limiting 
membrane—a distinct interspace separating the plate from the perichordal sheath. Four 
rami of the same chitinous substance project, one pair dorsally and one pair ventrally, 
and are well seen in sections through the otocystic region. 
Branchial System.—The head of the Teleostean embryo consists, as already indi- 
cated, of an expanded mass, chiefly neurochordal, or rather brain-tissue, and separated 
from the cortex of the yolk below by a thin layer of hypoblast (hy, Pl. III. fig. 1). The 
hypoblast forms here the roof of the sub-oral cavity, which has no floor, or rather, its floor 
is simply the periblast enveloping the yolk. Behind and below the ears a large oval area 
is apparently pushed in, resulting in the perforation of the lateral epiblast on each side of 
the otocystic region, these fenestrae (poa) communicating with the primitive mouth- 
chamber within (Pl VIII. figs. 3, 4). This opening, which may be called a primitive 
opercular opening (pow), though the true operculum is a new and later growth, is plairly 
visible in Molva vulgaris on the fourth day, along with a number of superficial irregu- 
larities, doubtless connected with the active changes going on at this point in connection 
with the branchial arches (Pl. X. fig. 6). The significance and function of this cleft 
(Spritzloch) upon each side is not readily understood, as the cesophageal lumen is not 
apparently open in front, and any perivitelline fluid which gains access to the sub-cephalic 
chamber, probably cannot find passage into the alimentary canal. HorrMan, however, 
speaks of it as produced by an evagination of the cesophagus, at first below the otocyst, 
but shifting forward and opening in front of the ear (No. 69, p. 7; vide his pl. i. fig. 5, 
emb. sp., also fig. 3, on p. 7). These embryonic “Spritzlocher,” he says, are merely 
‘transient structures, and the interesting question is raised as to whether they may be a 
reminiscence of the outer or extra-branchial system of the Cyclostomes, of which traces 
