214 Mr. F. Nansen on the Histological Structure 



rounds the true nervous '■'■ hyaloplasmatic'''' fibrillcR^ which are 

 generally nearly uncoloured. 



Besides the fibrillar web we also find in the fibrillar mass 

 percurrent nerve-cylinder s^ which usually appear to start 

 from ganglion-cells and run to peripheral nerves. These per- 

 current nerve-cylinders are, however, difficult to observe ; 

 they are certainly of greater calibre, but they generally be- 

 come stained by the same process as the fibrillar network, 

 and as their course is not always the same it is difficult to 

 trace them far. For shorter distances, however, they may be 

 traced, and there can be no doubt of their actual presence ; 

 we can even in some places perceive a tendency to the forma- 

 tion of bundles or larger cords of fibrils. Percurrent nerve- 

 cylinders, the exclusive destination of which would be the 

 establishment of a general communication, I have not been 

 able to demonstrate with certainty in the brain, although there 

 is every probability of their existence here also. In the 

 dorsal ganglionic cord, however, I have unmistakably seen 

 such. 



The nerve-cells in the brain of the Ascidia are of the most 

 different appearance and size. Those which especially strike 

 the eye in a section through an Ascidian brain are those of 

 the outer layer which surrounds the central fibrillar mass. 

 The cells of this layer are divided by Van Beneden and Julin* 

 into three categories. The smallest occur furthest in, in im- 

 mediate contact with the fibrillar mass ; the largest occur 

 exclusively in the periphery of the organ, and those of me- 

 dium size between these two. They say : " Tandis que les 

 petites et les cellules ganglionnaires moyennes constituent 

 autour de la masse ponctu^e une couche continue nettement 

 delimit^e, les grandes cellules ne se rencontrent qu'en certains 

 points " 



2. I find that this description applies pretty well, at least 

 in its main features, to the species examined by me. We 

 certainly find cells of all possible sizes from the smallest to 

 the largest ; but we shall always see that these last are gene- 

 rally situated outermost, while the smallest for the most part 

 lie innermost. Among these cells I have found both multi- 

 polar and unipolar forms ; but the latter are beyond comparison 

 tlie most general ; the large cells especially appear to be 

 principally unipolar. Among all these cells of such different 

 sizes in this outer layer I have found both forms of nervous 

 processes represented ; there are cells with processes which 

 pass directly to form peripheral nerve-cylinders, and others 

 with processes which divide up and lose themselves in 

 ♦ Loc. cit. p. 3.32. 



