766 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 



(Plate 37, fig. 46 C, z.}, which is the homologue of a similarly- 

 situated commissure present in the Elasmobranch brain 1 , while 

 behind the pineal gland is placed the posterior commissure. The 

 sides of the thalamencephalon are greatly thickened, forming 

 the optic thalami (Plate 37, figs. 46 C and D, op.th^, which are 

 continuous in front with the thickened outer walls of the hemi- 

 spheres. Below, the thalamencephalon is produced into a very 

 elongated infundibulum (Plate 37, 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 infe- 

 riores (Plate 37, fig. 46 E, l.in.}, which are continued posteriorly 

 into the crura cerebri. 



The pituitary body 2 (Plate 37, 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 37, 

 fig. 46 D, op. c/i.}, which is developed very early. It is A as stated 

 by Miiller, a true chiasma. 



The mid-brain (Plate 37, figs. 44 and 45, m. b.} is large, and 

 consists in both stages of (i) 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 37, figs. 46 

 E, F, G, op. /.) 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 37, fig. 46 E). 

 These masses probably become in the adult the more conspicuous 



1 Vide F. M. Balfour, Comparative Embryology, Vol. n. pp. 355 6 [the original 

 edition], where it is suggested that this commissure is the homologue of the grey 

 commissure of higher types. 



2 We have not been able to work out the early development of the pituitary body 

 as satisfactorily as we could have wished. In Plate 37, fig. 40, there is shewn an 

 invagination of the oral epithelium to form it ; in Plate 37, figs. 41 and 42, it is repre- 

 sented in transverse section in two consecutive sections. Anteriorly it is still con- 

 nected with the oral epithelium (fig. 41), while posteriorly it is free. It is possible 

 that an earlier stage of it is shewn in Plate 36, 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. 



