244 DR WALTER H. GASKELL. 



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

 consequence of which it becomes divided into two parts. The 

 lining walls of this fold form a double row of cells, the nuclei of 

 wliich 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. 



lieichenbach is unable to say that this secondary fold is 

 coincident with the primary involution, and therefore that the 

 junction between the two rows of large light 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 

 Eeichenbach's description in the crayfish, and says that he 

 finds only a thickening, 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 enough 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 an 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 Peichenbach's outer and inner 

 walls is not the line of junction between the retina and the 

 retinal ganglion, as Eeichenbach was inclined to think, but 

 rather a separation of two rows of large ganglion cells belonging 

 to the retinal ganglion. 



Unfortunately neither Eeichenbach nor Parker have traced 

 the development up to the adult stage, so that it is not possible 

 to be sure which layers in the fully-developed eye are the out- 

 come of this remarkable double band of large, lightly-staining 

 cells. In Eeichenbach's paper his figs. 224 and 225 represent 

 sections through the most advanced stage observed by him, and 

 \ cannot help agreeing with Parker that these figures do not 

 necessarily bear out Eeichenbach's contention ; in addition to 



^ Bulletin of the Harvard Musctim of Cnmp. Zoal., vol. xx. p. 1, 1890. 



