74 BULLETIN OF THE 
layer in Perca fluviatilis. Its position would be between the external 
nuclear layer and the layer nl. ba. (Figs. 20 and 21). The outer of the 
two “ Etagen” of Miiller’s tangential fulcrum cells appears to corre- 
spond 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. ba. and st. rtl. ex. of the Typhlogobinus retina the 
membrana pérforata and the stratum lacunosum, respectively. An 
objection to this interpretation is possibly presented by Hoffmann’s ac- 
count of the development of the Salmon retina. His Figures 10, 11, 
und 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 st. rtl. ex. The chief point to be made in this discussion of the 
homologies of the retinal layers is this. In the most differentiated 
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 the 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 somewhat 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. 
