DEVELOPMENT AND LIFE-HISTORIES OF TELEOSTEAN FISHES. 759 
ventricle. The structure thus formed is the hollow infundibulum (inf, Pl. XXIV. 
fig. 1), which abuts on the roof of the oral cavity below, though the two remain 
separate. The anterior part of this basal region gives origin to the optic nerves, which 
will be considered under the sense-organs. A chamber, or rather a loose meshwork of 
cells (Pl. XXIV. fig. 1), most probably hypoblastic, though possibly mesoblastic, lies 
behind the infundibulum, and into this loose mass the oral end of the notochord (no) 
pushes as it bends downward. In some sections the notochord and infundibulum are 
brought into closer contact. The elevation of the oral roof too is very distinctly marked at 
this time, and such probably (see Pl. XXIV. figs. 5, 6) corresponds to the curvature pro- 
duced in Elasmobranchs by the acute cranial flexure characteristic of those fishes and higher 
forms. On the summit of this arch a mass of cells appears, evidently a proliferation of 
the oral roof-cells rather than a diverticulum. This ovoid mass is the pituitary body 
(pt, Pl. XXIV. fig. 1). It lies in front of the infundibulum, and from its origin is in 
close relation to the base of the thalamencephalon. The precise origin of this body in 
these forms is difficult to make out, but its cells, as Horrman has clearly shown in the 
salmon and trout, are indistinguishable from the oral epithelium.* A small median 
swelling, not unlike the hypophysis in structure, lies in front of the latter—that is, behind 
and slightly under the point where the optic nerves decussate. When further advanced 
such appears to form the hypoaria or lobi inferiores—so well developed in Percoids, and 
their special ventricles in the adult communicate with the lumen of the infundibulum. 
The anterior fore-brain and the mid-brain at a very early stage so far overlap the 
intervening mass (the thalamencephalon) that only a small portion of its roof is super- 
ficially exposed (Pl. XXIV. fig. 1). This small extent of roof becomes very thin, as does 
also the roof of the anterior fore-brain, and it is much folded. In a transverse section 
through the mid-portion of the thalamencephalon before its walls have thinned out, a 
central aggregation of cells can be distinctly observed, and this soon exhibits a marked 
concentric arrangement, and become slowly pushed out as a papilliform process (PI. VIII. 
fig. 6). A lumen develops at a later time, and it communicates with the (third) 
ventricle below. Its cells, which were rounded and not dissimilar to the adjacent cells of 
the thalamencephalic roof at this time, assume a columnar disposition, and it now forms 
that very prominent and interesting structure in young fishes—the pineal gland. The 
primary rounded or conical form is not long retained ; it either becomes truncated, i.e., 
depressed, or more or less plicated, and pressed against the thin developing arachnoid 
membrane, which alone separates it from the integument. In the salmon and trout 
HorrMan gives a slightly different account of its origin. It arises, he says, as a true 
evagination, not a solid protuberance, and its lumen is continuous with a portion of the 
ventricle below distinctly marked off as a special recessus infra-pinealis (No. 69, 
pp. 100, 102). Moreover, its cells are at first epithelial in character and columnar, 
* Dourn states that the hypophysis makes its first appearance at the same time as the endodermal evaginations 
of the oral and branchial clefts. It arises as a pair of more or less distinct pouches much anterior to the paired oral 
slits. 
