74 BULLETIN OF THE 



layer in Perca fluviatilis. Its position would be between the external 

 nuclear layer and the layer 7il. ha. (Figs. 20 and 21). The outer of the 

 two " Etagen " of Miiller's tangential fulcrum cells appears to corro- 

 si)ond to Krause's membrana perforata, and likewise to M. Schultze's 

 basal plexus, and the inner to Krause's stratum lacunosum ; this would 

 make the layers nl. ha. and st. rtl. ex. of the Typhlogobinus retina the 

 membrana perforata and the stratum lacunosum, res})ectively. An 

 objection to this interpretation is possibly presented by Hoftniann's ac- 

 count of the development of the Salmon retina. His Figures 10, 11, 

 and 12 (Taf. V.) show that what he calls the tangential fulcrum cells 

 become differentiated quite early, certainly as early as the stage of 

 development represented by the partially developed retina of the adult 

 Typhlogobius. But judging from the position of this layer in relation 

 to the inner nuclear layer and the layer that he regards as the outer 

 granular layer, it would seem that his tangential fulcrum cells corre- 

 spond to the inner " Etagen " only of what Miiller designates by that 

 name. But according to my interpretation these cells are not present 

 in Typhlogobius, unless they be represented by the scattered cells in 

 layer .<f^ rtl. ex. The chief point to be made in this discussion of the 

 homologies of the retinal layers is this. In the most dilferentiated 

 retina, even though all the layers found in the normal adult fish eye 

 may be marked out, the differentiation is much less complete as regards 

 the zone between the two nuclear layers than it is in the normal eye of 

 a closely related genus ; while in a majority of individuals development 

 is arrested at a considerably earlier stage. 



In view of the almost universal statement that the rods and cones 

 are the latest of all the parts of the retina to be developed, it would, I 

 think, hardly be expected that the rods should be as complete as they 

 are in these eyes. O. Hertwig ('90, p. 402) says, " Of all parts of the 

 retina the remarkable rods and cones are tlie latest developed." Hoff- 

 mann ('83, p. 68) says, "According to all other authors [Lowe ex- 

 cepted] they [the external members of the rods and cones] arise latest 

 of all the retinal elements in the different animals ; and it is likewise so 

 in bony fishes." It would certainly seem that the testimony of the eyes 

 of Typhlogobius is against the absolute correctness of these statements. 



The recent papers of Hess ('89), Kohl ('89), and Schlampp ('91 and '92) 

 together with the somewliat older contributions to the same subject by 

 Leydig, Kadyi, Ciaccio, and others, make possible a detailed comparison 

 of the eyes of Typhlogobius with those of Proteus anguineus and Talpa 

 europea. 



