DEVELOPMENT AND LIFE-HISTOEIES OF TELEOSTEAN FISHES. 759 



ventricle. The structure thus formed is the hollow infundibulum (inf, PI. 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, w T hich 

 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 PI. 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, PI. 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 Hoffman 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 (PI. XXIV. fig. l). 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 (PL 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 

 Hoffman 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, 



* Dohbn 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. 



