390 W. H. GASKELL. 



and then in a subsequent section treat of the supra-oesophageal 

 ganglia. 



The intimate structure of the brain has been described so 

 fully and well by Ahlborn (3) that it is unnecessary for me to 

 treat of the different parts, section by section, as he has done. 

 I will therefore confine myself to the description of general 

 features as much as possible. 



In the whole of the epichordal portion the white and grey sub- 

 stances form two well-defined layers, the former being compact, 

 dense, and fibrous, while the latter is loose, transparent, and 

 composed mainly of cellular elements. Further, the grey 

 matter is itself divided into two very distinct layers in all 

 AmmoccBtes which are at all fully grown. The innermost of 

 these two layers is formed by the columnar cells which con- 

 stitute the lining epithelium of the central cavities ; they 

 are separated in the most marked way from the cells of the 

 outermost layer of the grey matter, not only by difference of 

 structure, but also by a distinct band of tissue, which presents 

 an appearance as of a limiting membrane. In sections 

 stained with hsematoxylin the line of separation between the 

 columnar cells with their dark-staining oval nuclei, and the 

 cells of grey matter with round nuclei which stain less darkly, 

 is very well shown. In fig. 4 I give a section through the 

 commencement of the fourth ventricle, and in fig. 5 a 

 magnified representation of a part of the same section. In 

 fig. 7 I give a magnified section through a portion of the 

 cerebral hemispheres. In both figures we see how clearly the 

 lining layer of columnar cells forms a distinct membrane, 

 which is marked off from the rest of the grey matter by a 

 clear space. This clear space separates the oval nuclei of the 

 lining epithelial cells from the neighbouring mass of cells 

 with round nuclei which form the greater portion of the grey 

 matter ; it looks as though the walls of the cells of the internal 

 membrane were distended with some transparent substance. 

 The nature of the contents of these cells is shown by treat- 

 ment with osmic acid. I have cut a continuous series through 

 the brain of Ammocoetes at different ages after the whole head 



