242 DR WALTER H. GASKELL. 



stanz. A comparison of this innermost part of the retina 

 (A, fig. 4), with the corresponding part in Berger's picture 

 of Musca (n.l.o.g., fig. 1), shows a ' most striking similarity 

 between the two. In both cases the fibres of the optic nerve 

 (O.U., fig. 1) which cross at their entrance pass into the 

 Punktsubstanz of this part of the retinal ganglion, and are 

 connected probably (though that is not proved in either case) 

 with the cells of the ganglionic layer. In both cases we 

 find two well-marked parallel rows of cells in this part of the 

 retina, of which one, the innermost, is composed in Ammocoetes 

 of large ganglion cells, and the other mainly of smaller, deeper 

 staining cells apparently supporting in function ; similarly, also, 

 in Branehipus, as I conclude from my own observations as well 

 as those of Berger and Glaus, the ganglionic layer is composed 

 partly of true ganglion cells and partly of supporting cells 

 arranged in a distinct layer. This part, then, of the retina of 

 Ammocoetes is remarkably like that of a typical Arthropod 

 retina, and forms that part of the retinal ganglion which may 

 be called the ganglion of the optic nerve. 



Next comes the ganglion of the retina (B, fig. 4) (Parker's 

 first optic ganglion), the cells of which form the small bipolar 

 granule cells of the inner nuclear layer ; granule cells arranged 

 in rows just as they are shown in Glaus' picture of the same 

 layer in the retina of Branehipus (fig. 2), just as they are 

 found in the cortical layers of the optic ganglion of the pineal 

 eye (ganglion habenula;), in the optic lobes and other parts of 

 the Ammocoetes brain, or in the cortical layers of the optic 

 ganglia of all arthropods. 



Between this small-celled nuclear layer (4, fig. 4) and the 

 layer of nuclei of the visual rod cells (7, fig. 4) (the external 

 nuclear layer), we find in the eye of Ammocates and Petro- 

 myzon two well-marked rows of cells of a most striking- 

 character — viz., the two remarkably regular rows of large 

 ephithelial-like cells with large conspicuous nuclei, which 

 give the appearance of two opposing rows of limiting epi- 

 thelium (5, fig. 4), already mentioned in connection with the 

 researches of Langerhans and W. Midler. Here, then, is a 

 striking peculiarity of the retina of the lamprey, and according 

 to Miiller the obliteration of these two layers can be traced as 



