182 PROCEEDINGS OF THE ACADEMY OF [1883. 



been observed in some animals. It is composed of a solid 

 mass of granular cells, traversed by numerous blood-vessels, and 

 resembles in structure, although more compact, one of the ordi- 

 nary Ij-mphatic glands. 



The lumen of the infundibulum becomes narrower before it 

 comm.unicates with the diacoelia, and the lateral walls become 

 thickened into two solid oval masses, largel}' composed of nerve- 

 cells. These bodies resemble the lobi inferiores of the Teleosts, 

 and, according to Stieda,' correspond in position with the tuber 

 cinereum of the mammalia ; anteriorly they gradually converge 

 (figs. 4 and 5), finally entering the thalami. At this point the 

 diacoelia has a cruciform shape, the lateral cavities separating the 

 tuber cinereum from the walls of the Diencephalon above. In 

 front of this is the thickening of the optic chiasma, and around 

 the upper portion of the ventricle is a row of compact cells which 

 resemble columnar epithelium. Anteriorly the latter flatten out, 

 covering a lateral expansion of the ventricle. Above this is a 

 small hollow sphere formed of a single layer of cells (fig. 7, a;); 

 the meaning of this structure is not known, and no mention of it 

 has been found by the writer elsewhere. It corresponds in 

 position with the external markings noticed upon the dorsal 

 surface of the Menopoma brain at this point (see fig. C, Di. f.). 

 Immediately below this point is a transverse band of nerve-fibres 

 which probably belong to the optic chiasma. 



The roof of the Diencephalon is of irregular thickness; forward 

 it is carried as a very thin lamina over the jiineal gland. The 

 structure of this body is nothing more than a rich plexus of blood- 

 vessels produced from the choroid ; in the apex are numerous fine 

 nuclei, resembling those of connective tissue, certainly not of 

 nerve-tissue. There is no evidence that the latter is present. 



It will thus be seen that the pineal Ixidy is a simple vascular 

 structure, properly speaking, in communication with the brain 

 cavity, since it is apparently surrounded by the brain parietes. 

 The pituitary body, on the other hand, is a compact glandular 

 structure, not in apparent communication with the brain cavity., 

 except l:)y an improbable process of osmosis through the attached 

 cells. 



1 Stud, iiber d. centrale Nervensystcm d. Knochenflscher. Zeits, ftir 

 wiss. Zoologie, Band xviii. 



