STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 
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of being directed forwards, as in most Vertebrate types, tends somewhat backwards, and 
rests on the mid-brain behind (Plate 24, figs. 44, 45, and 46 C and D, pn.). The roof 
of the thalamencephalon immediately in front of the pineal gland forms a sort of 
vesicle, the sides of which extend laterally as a pair of lobes, shown in transverse sec¬ 
tions in Plate 24, figs. 46 C and D, as th.l. This vesicle becomes, we cannot doubt, the 
vesicle on the roof of the thalamencepbalon wbicb we have described in the adult brain. 
Immediately in front of the pineal gland the roof of the thalamencephalon contains a 
transverse commissure (Plate 24, fig. 46 C, z.), which is the homologue of a similarly 
situated commissure present in the Elasmobranch brain,* while behind the pineal gland 
is placed the posterior commissure. The sides of the thalamencephalon are greatly 
thickened, forming the optic thalami (Plate 24, figs. 46 C and D, op.th.), which are 
continuous in front with the thickened outer walls of the hemispheres. Below, the 
thalamencephalon is produced into a very elongated infundibulum (Plate 24, figs. 44, 
45, 46 E, in.), the apex of which is trilobed as in Elasmobranchii and Teleostei. The 
sides of the infundibulum exhibit two lobes, the lobi inferiores (Plate 24, fig. 46 D, 
l.in.), which are continued posteriorly into the crura cerebri. 
The pituitary bodyt (Plate 24, figs. 44, 45, 46 E, pt.) is small, not divided into lobes, 
and provided with a very minute lumen. 
In front of the infundibulum is the optic chiasma (Plate 24, fig. 46 D, op.ch.), which 
is developed very early. It is, as stated by Muller, a true chiasma. 
The mid-brain (Plate 24, figs. 44 and 45, m.b.) is large, and consists in both stages 
of (1) a thickened floor forming the^ crura cerebri, the central canal of which 
constitutes the iter a tertio ad quartum ventriculum; and (2) the optic lobes (Plate 24, 
figs. 46 E, F, G, op.l.) above, each of which is provided with a cavity continuous with 
the median iter. The optic lobes are separated dorsally and in front by a well-marked 
median longitudinal groove. Posteriorly they largely overlap the cerebellum. In the 
anterior part of the optic lobes, at the point where the iter joins the third ventricle, 
there may be seen slight projections of the floor into the lumen of the optic lobes 
(Plate 24, fig. 46 E). These masses probably become in the adult the more conspicuous 
prominences of the floor of the ventricles of the <jptic lobes, which we regard as 
homologous with the tori semicirculares of the brain of the Teleostei. 
The hind-brain is formed of the usual divisions, viz. : cerebellum and medulla 
oblongata (Plate 24, figs. 44 and 45, cb., mcl). The former constitutes a bilobed projection 
of the roof of the hind-brain. Only a small portion of it is during these stages left 
* Vide F. M. Balfour, ‘ Comparative Embryology,’ vol. ii., pp. 355-6, where it is suggested that this 
commissure is the homologue of the grey commissure of higher types. 
f We have not been able to work out the early development of the pituitary body as satisfactorily as 
we could have wished. In Plate 24, fig. 40, there is shown an invagination of the oral epithelium to form 
it; in Plate 24, figs. 41 and 42, it is represented in transverse section in two consecutive sections. 
Anteriorly it is still connected with the oral epithelium (fig. 41), while posteriorly it is free. It is 
possible that an earlier stage of it is shown in Plate 23, fig. 35. Were it not for the evidence in other 
types of its being derived from the epiblast we should be inclined to regard it as hypoblastic in origin. 
