( 91 ) 



in which the structure of both cells was very clearly to he seen, 

 the two type* still present some marked differences, both in the 

 nuclei and in the structure of the protoplasm and the differentiations 

 on the surface of the cells. According to Joseph the nuclei of the 

 dorsal cells and of the ventral eve-cells possessed a similar granular 

 structure, differing from that of the other nerve-cells. In some cases 

 this is true, but in other cases the same structure is found in the 

 nuclei of other cells, and, when we examine a number of prepara- 

 tions, the structure of the nuclei both of the dorsal cells, of the 

 ventral light-percepting cells and of the other nerve-cells presents 

 so many differences and varieties, that there cannot be drawn any 

 conclusion out of that. Hut the capital difference between the two 

 cell-forms lies in the absence of a pigmented cap-cell in the dorsal 

 cells and the totally different form and structure of the two types. 



The light-percepting cells of the spinal cord possess a border of 

 short minute rods, lying close against the cap-shaped pigment cell. 

 The processus of the dorsal cells are much longer, and not rods, 

 but exactly shaped like hairs, or cilia. These hairs (lig. 8- 11) are 

 rather long, slender and thickly set and their course is often more 

 or less wavy. On the same cell they seem to be all of about the 

 same length; the hairs on different cells do not vary much in length. 



The ventral light-percepting cells are all of the same regular form. 

 The dorsal cells however present the most different forms. ') Some 

 are rather regular (tig. 8), some are long and slender (tig. 9), some 

 are of a very irregular shape, but in most cases these cells, when 

 we reconstruct them from the thin sections, appear to have a very 

 typical cup-shape. In tig. 10 I have drawn the median section 

 through one of these cups, in which the central hole in the cell is 

 figured, in tig. Ji such a cup-shaped cell is cut vertically to the 

 axis of the cup. 



These cells are surrounded by a glious basket of closely interwoven 

 fibres (in 'the figures this network is represented by a dark colour) 

 and the cells seem lo fill up the room left by this basket so that 

 between the surface of the cell and the inside of the basket there 

 remains an open space, in which the hair-like processes of the cell- 

 ' surface are seen. In well-preserved sections this space has the same 

 width on all sides of the cell, where the surface carries the hair-like 

 structures. . The hairs reach from the surface of the cell nearly to 



l ) Only such cells are described here, which seemed to be perfectly preserved 

 All those cells, of which the irregular form seemed to be caused by bad preser- 

 vation, are left out of the discussion. 



6 



Proceedings Royal Acad. Amsterdam. Vol. X. 



