THE EVIDENCE OF THE ORGANS OF VISION 99 



according to whom the earliest sign of the formation of the retina is an 

 ectodermic involution (Augen-einstulpung), which soon closes, so that 

 the retinal area appears as a thickening. In close contiguity to this 

 thickening, the thickening of the optic ganglion arises, so that that 

 part of the optic ganglion which will form the retinal ganglion fuses 

 with the thickened optic plate and forms a single mass of tissue. 

 Later on a fold (Augen-falte) appears in this mass of tissue, in conse- 

 quence of which it becomes divided into two parts. The lining walls 

 of this fold form a double row of cells, the nuclei of which are most 

 conspicuous because they are larger and lighter in colour than the 

 surrounding nuclei, so that by this fold the retina is divided into an 

 outer and an inner wall, the line of demarcation being conspicuous by 

 reason of these two rows of large, lightly-staining nuclei. 



Eeichenbach is unable to say that this secondary fold is coincident 

 with the primary involution, and that therefore the junction between 

 the two rows of large pale nuclei is the line of junction between the 

 retinal ganglion and the retina proper, because all sign of the primary 

 involution is lost before the secondary fold appears. 



Parker compares the appearances in the lobster with Reichenbach's 

 description in the crayfish, and says that he finds only a thicken- 

 ing, no primary involution ; at the same time he expressly states 

 that in the very early stages his material was deficient, and that he 

 had not grounds sufficient to warrant the statement that no involution 

 occurs. He also finds that in the lobster the ganglionic tissue which 

 arises by proliferation is divided into an outer and inner part ; the 

 separation is effected by a band of large, lightly-staining nuclei, which, 

 in position and structure, resemble the band figured by Eeichenbach. 

 According to Parker, then, the line of separation indicated in the 

 development by Reichenbach's outer and inner walls is not the line 

 of junction between the retina and the retinal ganglion, as Reichen- 

 bach was inclined to think, but rather a separation of two rows of 

 large ganglion-cells belonging to the retinal ganglion. 



The similarity between these conspicuous layers of lightly- 

 staining cells in Ammoccetes and in crustaceans is remarkably close, 

 and in both cases observers have found the same difficulty in inter- 

 preting their meaning. In each case one group of observers looks 

 upon them as ganglion-cells, the other as supporting structures. 

 Thus in the lamprey, Muller considers them to belong to the support- 

 ing elements, while Langerhans and Kohl describe them as a double 



