, 391 ) 
a stay at the Zoological Laboratory of Prof. Sr. Ap<rny in Kolozsvár 
once again L took up the theme, with some excellently fixed material 
I got through Prof. Aparny’s kindness. Finally the researches were 
carried on in the Histological Laboratory of Amsterdam. 
A. The structure of the light-percepting cells (eve-cells). 
In 1898 Hesse *) showed, that the peculiar pigmented cells, which 
are found in the spinal cord grouped round the ventral wall of the 
central canal, and which after beginning at the third segment, are 
arranged segmentally through the whole medulla, are each of them 
composed of two cells, one of them a big ganglioncell without pigment, 
the other a cupshaped cell, filled up entirely with dark brown pig- 
mentgrains; the last cell covering the greater part of the firstnamed 
cell and hiding it from view. 
The big unpigmented cells Hessr called eye-cells, light-percepting 
cells, the cupshaped pigmented cells he called the pigmenteup (Pig- 
mentbecher), and the whole complex he compared with the cupshaped 
eyes of the Planarians, that are equally composed of two cells, and 
attributed to it the function of light-perception. 
The arrangement of these two-celled eyes in the spinal cord is 
strictly segmental. They begin in the fourth segment, with two eyes; 
from there each segment is furnished with about 25 eyes. In the 
region of the tail the number lessens, until a segment has only one 
eve or none at all. 
The eyes lying ventrally of the central canal are always looking 
down, their line of vision, if we may call it so, being directed to 
the ventral side of the animal, those at the left side of the central 
canal look upward and to the right, those at the right side look down 
and to the right. 
The pigmenteup consists of one cell, the nucleus, when distinguishable, 
Iving at the concave side of the cup. 
The eve-cell is coneshaped, the base being covered by the pigment- 
cup, the top being drawn out into a thin process. At the basal 
side (turned towards the pigmenteup) the protoplasm. is differentiated 
into a layer of fine small rods, placed at right angles to the cell- 
periphery, and continuing in the protoplasm as a network of very 
thin fibres. Another layer of minute rods may be seen close against 
the pigmenteup. Between those two layers a clear space is formed, 
Which is not caused by a shrinking of the cell. 
W. kKravse') did not agree with the results of Hesse. He still 
maintained that the pigmented cells in the spinal cord consisted 
1) Zeitschr. f. Wiss. Zoologie. Bd. 63. 1898. p. 456. 
1) Anat. Anzeiger. Bd. 14. Pag. 470. Zoöl. Anzeiger. Bd. 21. p. 481, 
