THE EYE OF PECTEN. 79 



Hesse finally noticed tlie resemblance of these outer inter- 

 stitial cells to tlie inner ganglionic cells of Patten, Rawitz and 

 Schreiner, and called all of them " Zwischenzellen/' stating- 

 at the same time that they did not bear exactly the appear- 

 ance of nerve-cells, but his preparations showed that the 

 fibres of the distal nerve arose from them. He did not regard 

 them as ganglion-celis but considered them to be optic sense- 

 cells. The function of the outer cells is not stated, but they 

 are not supposed to be connected with the distal branch of 

 the optic nerve. 



The next mention of these cells occurs in Schneider's 

 ' Text-book of Histology' (38). Schneider finds no connec- 

 tion existing between the " Zwischenzellen " and the distal 

 branch of the optic nerve, nor any junction of the latter with 

 the outer layer of cells, bu.t finds that the nerve-fibres pene- 

 trate between them and cannot be ti'aced further. He also 

 describes how at the edges of the retina the cells of the outer 

 layer at various places surround, collar-like, branches of the 

 nerve. I believe this (see his illustrations, p. 560) must have 

 been caused by artefacts. The interstitial cells are not con- 

 sidered to be sense-cells. 



In 1904 appeared Hyde's remarkable account of the nerve- 

 endings in the retina, which really caused my attention to be 

 drawn to the Pecten eye. Hesse had previously stated that 

 niethylene-blue methods had failed him, but that the problems 

 of the retina Avould in all pi-obability be solved by the attain- 

 ment of success with this stain. According to Hyde, methy- 

 lene-blue methods were perfectly successful and solved all, 

 the result being a description of the retina which stands in 

 striking opposition to all previous work. Hyde finds that the 

 inner interstitial cells are the nerve-cells connected with 

 the axial fibre of the rods, and only mentions one row 

 of outer cells which are supposed to be connected to the 

 fibres of the optic nerve. 



So much for the outer cells ; I shall have occasion to make 

 further reference to Hyde's work later. In 1908 Hesse refers 

 again to the Pecten eye (43), and now finds a connection 



