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-einstiilpung), 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. 
Reichenbach 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 Reichenbach. 
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, Miller considers them to belong to the support- 
ing elements, while Langerhans and Kohl describe them as a double 
